From 1946 to 1948, American public health doctors deliberately infected nearly 700 Guatemalans — prison inmates, mental patients and soldiers — with venereal diseases in what was meant as an effort to test the effectiveness of penicillin.

American tax dollars, through the National Institutes of Health, even paid for syphilis-infected prostitutes to sleep with prisoners, since Guatemalan prisons allowed such visits. When the prostitutes did not succeed in infecting the men, some prisoners had the bacteria poured onto scrapes made on their penises, faces or arms, and in some cases it was injected by spinal puncture.

If the subjects contracted the disease, they were given antibiotics.

“However, whether everyone was then cured is not clear,” said Susan M. Reverby, the professor at Wellesley College who brought the experiments to light in a research paper that prompted American health officials to investigate.

The revelations were made public on Friday, when Secretary of State Hillary Rodham Clinton and Health and Human Services Secretary Kathleen Sebelius apologized to the government of Guatemala and the survivors and descendants of those infected. They called the experiments “clearly unethical.”

“Although these events occurred more than 64 years ago, we are outraged that such reprehensible research could have occurred under the guise of public health,” the secretaries said in a statement. “We deeply regret that it happened, and we apologize to all the individuals who were affected by such abhorrent research practices.”

In a twist to the revelation, the public health doctor who led the experiment, John C. Cutler, would later have an important role in the Tuskegee study in which black American men with syphilis were deliberately left untreated for decades. Late in his own life, Dr. Cutler continued to defend the Tuskegee work.

His unpublished Guatemala work was unearthed recently in the archives of the University of Pittsburgh by Professor Reverby, a medical historian who has written two books about Tuskegee.

President Álvaro Colom of Guatemala, who first learned of the experiments on Thursday in a phone call from Mrs. Clinton, called them “hair-raising” and “crimes against humanity.” His government said it would cooperate with the American investigation and do its own.

The experiments are “a dark chapter in the history of medicine,” said Dr. Francis S. Collins, director of the National Institutes of Health. Modern rules for federally financed research “absolutely prohibit” infecting people without their informed consent, Dr. Collins said.

(Read by Nelly Min. Nelly Min is a journalist at the China Daily Website.)