The retirement was announced by MLB via the following statement (courtesy of ESPN.com news services)::
"Major League Baseball recently notified Manny Ramirez of an issue under Major League Baseball's Joint Drug Prevention and Treatment Program," the statement said. "Rather than continue with the process under the Program, Ramirez has informed MLB that he is retiring as an active player. If Ramirez seeks reinstatement in the future, the process under the Drug Program will be completed. MLB will not have any further comment on this matter."
Although the message is open to interpretation, it seems that Manny has once again tested positive for a banned substance and, rather than face the embarrassment of another suspension, has decided to walk away from the game.