The United States government has said sorry for deliberately infecting hundreds of Guatemalans with syphilis as part of medical experiments six decades ago.
None of the mentally ill patients and prisoners who had given their consent.
Guatemala’s president, Alvaro Colom. described the experiments as a crime against humanity and refused to rule out legal action.
President Barack Obama personally called his Guatemalan counterpart to apologise but made no offer of compensation.
The tests were uncovered by an American professor earlier this year.
It echoes a 1960 experiment called the Tuskegee study on black American men who deliberately left untreated for syphilis.
Syphilis is a sexually transmitted infection that can cause heart problems, blindness, mental illness and even death.