Washington ? Saying American soldiers should not be used as “guinea pigs for experimental drugs,” a federal judge Monday ordered the Pentagon to stop mandatory anthrax vaccinations started in 1998.

U.S. District Judge Emmet Sullivan said he was convinced by plaintiffs in a class action suit that the vaccine is experimental and being “used for an unapproved purpose” — that is, for exposure to airborne anthrax as well as exposure through the skin.

Officials at the Defense Department and Food and Drug Administration said they had not seen the ruling and had no immediate comment. But the federal government has long maintained that the licensed vaccine is safe, is not experimental and can be used for protection against anthrax inhaled or absorbed through the skin.

More than 900,000 servicemen and women have received the shots, among the millions of doses of various vaccines administered annually to protect troops against disease and bioterror threats. Hundreds of service members have been punished or discharged for refusing them, according to the Pentagon.

“The women and men of our armed forces put their lives on the line every day to preserve and safeguard the freedoms that all Americans cherish and enjoy,” Sullivan said in Monday’s 34-page ruling.

“Absent an informed consent or presidential waiver, the United States cannot demand that members of the armed forces also serve as guinea pigs for experimental drugs,” Sullivan said.

Sullivan ordered the government to file responses by Jan. 30.

Anthrax is a naturally occurring bacterium that typically affects sheep and cattle. When inhaled, dry anthrax spores can be deadly to humans.