The United States revealed on Friday that the government conducted medical experiments in the 1940s in which doctors infected soldiers, prisoners and mental patients in Guatemala with syphilis and other sexually transmitted diseases.

The experiments, led by a federal doctor who helped conduct the famous Tuskegee syphilis study in Alabama, involved about 1,500 men and women who were unwittingly drafted into studies aimed at determining the effectiveness of penicillin.

The tests, which were carried out between 1946 and 1948, infected subjects by bringing them prostitutes who were either already infected or purposefully infected by the researchers and by using needles to open wounds that could be contaminated.

"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," Secretary of State Hillary Rodham Clinton and Health and Human Services Secretary Kathleen Sebelius said in a joint statement apologizing for the experiments. "We deeply regret that it happened, and we apologize to all the individuals who were affected by such abhorrent research practices."

President Obama had been briefed about the revelations and called Guatemalan President Alvaro Colom to "personally express that apology," White House spokesman Robert Gibbs said."Obviously, this is shocking. It's tragic. It's reprehensible," Gibbs said.

The Guatemalan government planned to investigate, saying it "deeply deplores that these experiments affected innocent people," according to a statement issued late in the day.

In addition to exposing another episode of unethical medical experimentation, officials said the revelations were concerning because they could further discourage already often-suspicious minorities and others from participating in medical research. They also come as U.S. drug companies are increasingly going to poor, less-educated countries to test new drugs and other therapies.

"At a time when so much medical research is global, it behooves us to take account of what has been done in the past by American researchers in other countries," said Susan M. Reverby, a professor in the history of ideas and professor of women's and gender studies at Wellesley College in Massachusetts who discovered the experiments while investigating Tuskegee for a book.

In Tuskegee, perhaps the most notorious medical experiment in U.S. history, hundreds of African American men with late-stage syphilis were left untreated to study the disease between 1932 and 1972. In the Guatemala case, the subjects were treated, but it remains unclear whether they were treated adequately or what became of them.

Reverby discovered the experiments while reading papers in the University of Pittsburgh's archives from John C. Cutler, a doctor with the federal government's Public Health Service who later participated in Tuskegee. He died in 2003.

"I almost fell out of my chair when I started reading this," Reverby said in a telephone interview. "Can you imagine? I couldn't believe it."

The studies were sponsored by the Public Health Service, the National Institutes of Health and the Pan American Health Sanitary Bureau (now the World Health Organization's Pan American Health Organization) and the Guatemalan government. The goal was to assess whether taking penicillin right after sex would prevent sexually transmitted infections.