(CNN) -- Major League Baseball on Monday suspended 13 players, including New York Yankees star Alex Rodriguez, after an investigation into the use of performance-enhancing drugs.
The league suspended Rodriguez for 211 regular-season games through the 2014 season, but the 38-year-old slugger said he planned to appeal. Twelve other players have accepted 50-game suspensions without pay.
A-Rod and the other players are accused of having ties to the now-shuttered Biogenesis anti-aging clinic in south Florida and taking performance-enhancing drugs. Rodriguez has denied the accusation.
"I'm fighting for my life. I have to defend myself. If I don't defend myself, no one else will," he told reporters after the league announced its decision.
The last seven months have been a "nightmare," he said.
It "has been probably the worst time of my life for sure," said Rodriguez, "obviously for the circumstances that are at hand and also dealing with a very tough surgery and a rehab program, and being 38."
Asked directly whether he had used performance-enhancing drugs, he declined -- repeatedly -- to comment.
"I think we'll have a forum to discuss all of that, and we'll talk about it then," Rodriguez said.
Earlier in a written statement, he said that he was disappointed with the penalty and intends to appeal. He thanked family, friends and fans for their support and stressed that he was eager to get back on the field with his teammates.
His suspension is set to go into effect on Thursday, the league said. But officials also said that Rodriguez could keep playing if he appeals.
He arrived Monday in Chicago, where he played in a night game against the White Sox. The Yankees' roster listed him in the starting lineup, batting fourth and playing third base.
At his first at-bat, Rodriguez was met with boos and some cheers from the crowd. He hit a single to left field.
Before the game started, Yankees manager Joe Girardi said Rodriguez's suspension wouldn't affect the team.
"He's here. He's going to play," Girardi told reporters. "It really doesn't change anything for us."
Major League Baseball's sweeping investigation shows a change in how officials handle performance-enhancing drugs, CNN sports reporter Rachel Nichols said.
"It used to be that baseball protected its own," she said. "And now we're seeing other people in the clubhouse, from managers to the players, saying, 'You know what? Go after these guys. We don't want them in the game.'"
On Monday, Commissioner Bud Selig said the league had no choice but to investigate the allegations in order to "maintain integrity, fairness and a level playing field."
"Despite the challenges this situation has created during a great season on the field," he said, "we pursued this matter because it was not only the right thing to do, but the only thing to do."
Sports Illustrated writer Ben Reiter described the suspension of Rodriguez as "unprecedented."
"The sheer magnitude of the suspension is just one we've never seen before," he said.
Union: 'We agree with his decision to fight his suspension'
The league said Rodriguez's punishment is based on his alleged use and possession of banned performance-enhancing substances, including testosterone and human growth hormone, for multiple years.
Officials also accuse him of "engaging in a course of conduct intended to obstruct or frustrate" their investigation into the matter.
The Major League Baseball Players Association said Monday that it's standing behind Rodriguez.
"We agree with his decision to fight his suspension," the union said in a written statement, adding that it believed the league's commissioner "has not acted appropriately."
Rodriguez's lawyer, David Cornwell, criticized the league.
"Major League Baseball has gone well beyond the authority granted it in its Joint Drug Agreement and the Basic Agreement," he said. "Consequently, we will appeal the discipline and pursue all legal remedies available to Alex."
Before Monday's announcement, the Biogenesis scandal had already ensnared one star: 2011 National League MVP and Milwaukee Brewers outfielder Ryan Braun.
Last month, Braun was suspended without pay for the rest of this season for violating the league's drug policy.
Yankees: We're "focused on playing baseball"
The New York Yankees declined to comment on Rodriguez's suspension until the completion of the appeal process.
"In the meantime," the team said in a written statement, "the Yankees remain focused on playing baseball."
The team did say it was disappointed by the suspension of Yankees catcher Francisco Cervelli, who was one of the 12 players who agreed to the 50-game suspension.
"It's clear that he used bad judgment," the Yankees said.
Major League Baseball issued a statement saying that Rodriguez was being disciplined under both baseball's Joint Drug Prevention and Treatment Program and the players' collective bargaining agreement.
Under the drug program, "Rodriguez's suspension will be stayed until the completion of his appeal if Rodriguez files a grievance challenging his discipline," the MLB statement said.
That is also the usual case for suspensions related to drug offenses under the collective bargaining agreement, but the statement did not specifically address that aspect.
Impressive record
Rodriguez is considered one of the game's greatest sluggers. He has 647 home runs -- the fifth most ever -- in 19 seasons.
In 2009, he had an outstanding postseason as he helped the Yankees win their most recent World Series title.
He holds the largest contract ever in American sports, signing with the Yankees in 2007 for $275 million over 10 years.
Rodriguez said Friday he believes his contract makes him an attractive target for a baseball ban or suspension.
"There's more than one party that benefits from me not ever stepping back on the field -- and that's not my teammates and it's not the Yankees fans," he said.
Rodriguez has missed the entire 2013 season so far after undergoing hip surgery.
Other players suspended
The other players suspended by Major League Baseball Monday are:
Philadelphia Phillies pitcher Antonio Bastardo
San Diego Padres shortstop Everth Cabrera
New York Yankees catcher Francisco Cervelli
Texas Rangers outfielder Nelson Cruz
Padres pitcher Fautino De Los Santos
Houston Astros pitcher Sergio Escalona
Yankees outfielder Fernando Martinez
Seattle Mariners catcher Jesus Montero
Free agent pitcher Jordan Norberto
Detroit Tigers shortstop Jhonny Peralta
New York Mets outfielder Cesar Puello
Mets infielder/outfielder Jordany Valdespin
De Los Santos, Escalona, Martinez, Montero, Puello and Valdespin are currently on minor league rosters.
In written statements, several teams said they backed the league's decision.
Cruz said he had made an "error in judgment" after an infection left him concerned about whether he would be physically able to play.
"Faced with this situation, I made an error in judgment that I deeply regret, and I accept full responsibility for that error. I should have handled the situation differently, and my illness was no excuse," he said.
"I look forward to regaining the trust and respect of the Rangers organization, my teammates, and the great Rangers' fans, and I am grateful for the opportunity to rejoin the team for the playoffs."