The United States has apologised for infecting almost 700 people in Guatemala with gonorrhoea and syphilis during medical tests more than 60 years ago.
Secretary of State, Hillary Clinton, says the tests were unethical and abhorrent, and President Barack Obama has personally apologised to his Guatemalan counterpart.
The BBC reports many of the almost 700 infected were mentally ill patients and prisoners - none of whom gave their consent.
At the time, the US wanted to test vaccines against venereal diseases.
Mr Obama told President Alvaro Colom the experiments ran contrary to American values.
In his phone call to the Guatemalan president, Mr Obama reaffirmed the United States' unwavering commitment to ensure that all human medical studies meet exacting US and international legal and ethical standards, the White House says.
President Obama also "underscored the United States' deep respect for the people of Guatemala and the importance of our bilateral relationship".
The US has promised an investigation.
Mr Colom says he was very angry and upset about the trials which he described as a crime against humanity.
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 Professor Susan Reverby at Wellesley College. She says the Guatemalan government gave permission for the tests, which took place between 1946 - 1948.
Doctors used prostitutes with syphilis to infect the test subjects, as they tried to determine whether penicillin could prevent syphilis, not just cure it.
The BBC reports the patients were then treated for the disease, but it is unclear whether everyone was cured.
Previous experiment
Professor Reverby has previously done research on the Tuskegee experiment, where 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 - 1972, with President Bill Clinton eventually apologising for it.
