The main charge against Mr Karadzic concerns the 1995 Srebenica massacre
Lawyers for the former Bosnian Serb leader, Radovan Karadzic, have filed papers arguing that all charges against him should be dropped.
They say US diplomat Richard Holbrooke promised him immunity from prosecution on condition he gave up politics - something Mr Holbrooke strongly denies.
Mr Karadzic is on trial at the UN tribunal in The Hague, facing 11 charges including genocide.
The tribunal has said that any immunity deal would not be binding.
The tribunal says even if a deal had been agreed, the trial would proceed anyway.
Mr Karadzic's lawyers filed a motion of more than 100 pages, demanding the court hold a special hearing to establish the truth of his claims.
CHARGES AGAINST KARADZIC
Eleven counts of genocide, war crimes, crimes against humanity and other atrocities
Charged over shelling Sarajevo during the city's siege, in which some 12,000 civilians died
Allegedly organised the massacre of up to 8,000 Bosniak men and youths in Srebrenica
Targeted Bosniak and Croat political leaders, intellectuals and professionals
Unlawfully deported and transferred civilians because of national or religious identity
Destroyed homes, businesses and sacred sites
He says Mr Holbrooke, then the US envoy to Bosnia, agreed to provide him with immunity at a meeting in Belgrade on 18-19 July, 1996.
Mr Karadzic does not claim to have attended the meeting, but says the former Bosnian Serb assembly speaker, Momcilo Krajisnik, and foreign minister, Aleksa Buha, were there and could testify to Mr Holbrooke's alleged promise.
He also says he has testimony from other witnesses, including sources in the US state department, along with written documents and articles, to support his claim.
"The indictment should be dismissed, or the proceedings should be stayed, so that the hands of the tribunal are not stained with Holbrooke's deception," the motion said.
One of Radovan Karadzic's legal team, Peter Robinson, told the BBC: "The first step that we want to take is to ask that the trial not be held and the case be dismissed because of the promise that Richard Holbrooke made. If that's not successful, then we'll have to defend the case at trial, and Dr Karadzic is prepared to do that."
Mr Holbrooke - now the US envoy to Pakistan and Afghanistan - strongly denies any such deal, describing the claim as "laughable" and "a lie".
The BBC correspondent in The Hague, Geraldine Coughlan, says court observers are anxious to see whether judges will allow a special hearing, which Mr Karadzic's lawyers argue in is in the interest of "fundamental fairness".
Mr Karadzic was arrested and brought to the tribunal last year, after more than a decade in hiding.
He is accused of genocide over the massacre of Bosnian Muslims at Srebrenica in 1995, and is also charged with war crimes and crimes against humanity.
The court filed a not guilty plea on his behalf after he refused to offer a plea, saying the court lacked jurisdiction.