Spec. Sean Baker, 38, was assaulted in January 2003 after he volunteered to wear an orange jumpsuit and portray an uncooperative detainee. Baker said the MPs, who were told that he was an unruly detainee who had assaulted an American sergeant, inflicted a beating that resulted in a traumatic brain injury.

Baker, a Persian Gulf War veteran who re-enlisted after the Sept. 11 terror attacks, was medically retired in April 2004. He said the assault had left him with seizures, blackouts, headaches, insomnia and psychological problems.

In the lawsuit filed in U.S. District Court in Lexington, Ky., Baker demanded reinstatement in the Army in a position that would accommodate his medical disability. He said the Army put him on medical retirement against his wishes.

"Somebody has to step up to serve, and I still want to serve," Baker said Friday in a telephone interview from his home in Georgetown, Ky.

A Pentagon spokeswoman declined to comment, saying she had not seen the lawsuit and could not discuss pending litigation.

The Pentagon first said that Baker's hospitalization after the training incident was not related to the beating. Later, officials conceded that he had been treated for injuries suffered when a five-man MP "internal reaction force" choked him, slammed his head several times against a concrete floor and sprayed him with pepper gas.

Baker said he had put on the jumpsuit and squeezed under a prison bunk after being told by a lieutenant that he would be portraying an unruly detainee. He said he was assured that MPs conducting the "extraction drill" knew it was a training exercise and that Baker was an American soldier.

As he was being choked and beaten, Baker said, he screamed a code word, "red," and shouted: "I'm a U.S. soldier! I'm a U.S. soldier!" The beating continued, he said, until the jumpsuit was yanked down during the struggle, revealing his military uniform.

No one has been disciplined or punished for the assault, said Baker's attorney, T. Bruce Simpson Jr. Simpson said the Army's Criminal Investigation Division told him last month that it had completed an investigation and had referred it to the Army's legal section for review. A CID spokesman did not respond to a request for comment Friday.

Baker receives $2,350 a month in military disability benefits, plus $1, 000 a month in Social Security, Simpson said.

Separately Friday, the Pentagon announced that a subsidiary of Houston- based Halliburton had been awarded $30 million to build an improved 220-bed prison at Guantanamo.

Kellogg Brown and Root Services Inc. of Arlington, Va., is to build a two- story prison that includes day rooms, exercise areas, medical bays, air conditioning and a security control room, according to the Pentagon. It is to be completed by July 2006.

Sen. Frank Lautenberg, D-N.J., issued a statement criticizing the deal, calling Halliburton the "scandal-plagued former employer of Vice President (Dick) Cheney." Lautenberg has sought hearings into the contracts awarded to Halliburton for work in Iraq.