Testimony began today in U.S. District Court here in what is considered a major test case challenging the practice of using prison inmates to test new vaccines for infectious diseases including typhoid, cholera, malaria and influenza.

Attorneys for the National Prison project of the American Civil Liberties Union contend the practice is unconstitutional because inmates were "coerced"into "volunteering" for the testing.

The inmates were "coerced," the lawyers say, both by the $2 daily fee the project's managers offered and by the overcrowded conditions at the prison, which made inmates more anxious to take the chance of living in the relatively comfortable surroundings of the isolation ward where the experiments were conducted.

In addition, the attorneys claim, the inmates were not adequately informed about the program's risks.

But Dr. Richard B. Hornick, a former professor of medicine at the University of Maryland who ran the program at the Jessup jail, testified today that the conditions in the experimental ward -- which had air conditioning and a color TV -- and the $2 per day offered to inmates who volunteered for tests was not "coercion." If it was, he said, "everybody and their brother would have been volunteering and the number of inmates we had was unusually small.

"We just didn't have an exceptional plethora of inmates dying to get into our studies," Hornick said.

What is at issue in the trial this week is whether state correctional officials and doctors who administered a series of experimental vaccines to prisoners over an 18-year period violated the prisoners' constitutional rights to due process and equal protection of the law.

It will be up to Judge Frank A. Kaufman, who is presiding over the nonjury trial, to determine if prison conditions may have deprived inmates of the ability to honestly volunteer for the tests.

Attorneys on both sides say this may be the first suit in the country aimed at stopping the experimental vaccine testing program.

That issue may already be moot, however, since the attorneys also agree that prisoners are no longer used for federal experiments in testing new drugs.

Hornick said the infectious diseases under study at the Maryland jail dysentery, malaria, cholera, typhoid and influenza -- were "uniquely human" and vaccines could not be tested.

The prison population offered a "controlled environment" where doctors could study the effects of the vaccines on a long-term basis under controlled conditions, Hornick added.

The vaccines which were tested on prisoners are now being used in other countries and have successfully controlled some diseases in American servicemen stationed overseas, Hornick said.

Eight prisoners are named as plaintiffs in the suit, which has been certified as a class action for 513 inmates who participated in the testing from 1973 to Jan. 8, 1976, when the program in Maryland ended. Almost all inmates who participated in the program between 1958 and 1973 were not eligible to sue because of the statute of limitations.

One of the plaintiffs, Robert Morgan, who is now out of jail and studying law, said outside the courtroom today that he was "harassed" and denied certain prison privileges when he refused to participate in the testing.

Dr. William Woodward, one of the doctors who formerly administered the vaccines, said 3,656 prisoners participated in the vaccine testing from 1958 until 1976, or about 11 percent of the inmates. They came back repeatedly to the program so that each of those prisoners averaged participation in 3.4 experiments, Woodward added.

One of the Maryland inmates wrote a U.S. House of Representatives committee in 1975 begging them not to cut off funding for such experimental studies.