In a 77-page filing, the Departments of Defense and the Army said they oppose the defendants' request for a partial summary judgment in the case, Courthouse News Service reported.
Vietnam Veterans of American filed a class-action suit in 2009 charging that at least 7,800 soldiers were used as guinea pigs in an experiment known as Project Paperclip.
The soldiers said they were administered hundreds of drugs ranging from deadly Sarin to LSD for a study of substances to control human behavior.
Some soldiers died as a result and other suffered seizures and paranoia, the veterans said.
In September, U.S. District Judge Claudia Wilken granted the plaintiffs class action status making thousands of veterans eligible for relief.
"Although plaintiffs have styled much of this lawsuit as one challenging agency delay in the performance of a discrete legal obligation, it is undisputed that the government has engaged in decades-long efforts to reach out to test participants and assess their health," said attorney Joshua Gardner for the Justice Department.
A ruling on the request for a partial summary judgment in the case is scheduled for March 14.
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