US testing that infected hundreds of Guatemalans with gonorrhoea and syphilis more than 60 years ago was a "crime against humanity", Guatemalan President Alvaro Colom has said President Barack Obama has apologised for the medical tests, in which mentally ill patients and prisoners were infected without their consent.
Mr Obama told Mr Colom the 1940s-era experiments ran contrary to American values, Guatemala said.
The US has promised an investigation.
'Shocking, tragic, reprehensible'
Syphilis can cause heart problems, blindness, mental illness and even death, and although the patients were treated it is not known how many recovered.
Evidence of the programme was unearthed by Prof Susan Reverby at Wellesley College. She says the Guatemalan government gave permission for the tests.
No offer of compensation has yet been made, but an investigation will be launched into the specifics of the study, which took place between 1946 and 1948.
White House spokesman Robert Gibbs said on Friday the news was "shocking, it's tragic, it's reprehensible".
In an interview with the BBC, Mr Colom said the test subjects were "victims of rights abuses".
"There's been a very strong reaction in the Guatemalan media and by my compatriots," he said.
"Of course, there may have been similar incidents in other countries around the world, but speaking as the president and a Guatemalan, I would have preferred that these events had never happened on this soil."
The joint statement from Mrs Clinton and Health Secretary Kathleen Sebelius said: "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.
"We deeply regret that it happened, and we apologise to all the individuals who were affected by such abhorrent research practices."
In his phone call to President Colom, Barack Obama reaffirmed the United States' unwavering commitment to ensure that all human medical studies conducted today meet exacting US and international legal and ethical standards, the White House.
President Obama also "underscored the United States' deep respect for the people of Guatemala and the importance of our bilateral relationship".
Unaware
The study by Prof Reverby shows that US government medical researchers infected almost 700 people in Guatemala with two sexually transmitted diseases.
The patients - prisoners and people suffering mental health problems - were unaware they were being experimented upon.
The doctors used prostitutes with syphilis to infect them, or inoculation, as they tried to determine whether penicillin could prevent syphilis, not just cure it.
The patients were then treated for the disease, but it is unclear whether everyone was cured.
Prof Reverby has previously done research on the Tuskegee experiment, where the US authorities measured the progress of syphilis in African-American sharecroppers without telling them they had the disease or adequately treating it.
The experiment ran from 1932 to 1972, with President Bill Clinton eventually apologising for it.
