The US government is facing a potential lawsuit on behalf of Guatemalans who were deliberately given gonorrhoea or syphilis in medical tests decades ago.
Syphilis can cause blindness, insanity and even death
Lawyers have given the US authorities until Friday to find a way to settle claims out of court or face action.
Hundreds of Guatemalan prisoners and mentally ill patients were infected, without their consent, to study the effects of penicillin.
Last year, the US government apologised for the "reprehensible" experiments.
Lawyers representing several Guatemalans have written to US Attorney General Eric Holder asking for a way to resolve the issue out of court.
The lawyers said that they would file the lawsuit in federal court in Washington unless the Obama administration responded positively to their offer by Friday.
"We have decided to create one opportunity to see if we can settle the issues presented in this tragic situation without involving the judicial process," the letter from lawyers Andres Alonso and Terrence Collingsworth said.
The lawyers say their action could become a class-action lawsuit on behalf of more Guatemalans.
It is not clear how many plaintiffs there could be, as the lawsuit would include those infected but also their relatives and survivors.
The study carried out by US scientists took place in Guatemala between 1946 and 1948.
Evidence of the programme was unearthed by Prof Susan Reverby at Wellesley College in the US.
The Guatemalan government of the day gave permission for the tests, she said.
But the people infected, prisoners and those suffering mental health problems, were unaware they were being experimented on.
BBC News - Guatemalans threaten to sue US over sex infection tests
Guatemalans threaten to sue US over sex infection tests
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