Ex-Bosnian Serb army chief Ratko Mladic has refused to testify after former Bosnian Serb leader Radovan Karadzic called him as a defence witness at his war crimes trial at The Hague.
Denouncing the UN tribunal as "satanic", Mr Mladic said testifying could prejudice his own case.
Both men deny charges of war crimes and crimes against humanity.
They exchanged genial greetings in their first public appearance together since the 1990s Bosnia war ended.
As he left court, Mr Mladic said: "Thanks a lot, Radovan. I'm sorry, these idiots wouldn't let me speak. They defend Nato bombs."
'Good morning, general'
Mr Karadzic, 68, faces 11 charges, including genocide relating to the Srebrenica massacre in July 1995.
At Tuesday's hearing, his lawyer argued that Mr Mladic was "the one person in the whole world who knows best what happened in the war in Bosnia" and that Mr Karadzic was asking him to do his best to testify and to tell what had occurred.
Ratko Mladic, 71, initially refused to take the oath, saying: "Your subpoenas, your platitudes, your false indictments, I do not care one bit about any of it."
He added: "This is a satanic court, which is putting on trial us Serbs because we are defending our people from you."
The judge warned him he could be held in contempt, with a possible jail term of up to seven years.
The session was then adjourned, apparently so Mr Mladic's dentures could be retrieved from his cell.
The judge then advised Mr Mladic he was not obliged to answer questions if he thought the answers would incriminate him.
Mr Karadzic then addressed Mr Mladic in person, saying: "Good morning general, sir."
Mr Mladic did answer Mr Karadzic's first question - listing the posts and dates of his military career.
But following the second question - "Did you ever inform me that prisoners from Srebrenica would be, were being or had been executed?" - Mr Mladic said: "I refuse to testify on the grounds of my health and because it may prejudice my rights as an accused."
Lawyers representing Mr Mladic say he suffers from a memory disorder that makes it hard for him to differentiate between truth and fiction.
The judge ruled Mr Mladic would not be compelled to answer.
Mr Karadzic read out his remaining questions, but received the same reply.
Mr Mladic again asked if he could read out a seven-page statement - supported by Mr Karadzic - but was refused and the session was adjourned.
Mr Karadzic had been hoping his former ally's answers would support his claims that the orders to commit war crimes did not come from him, says the BBC's Anna Holligan in The Hague.
The key charges facing Mr Karadzic relate to Sarajevo and Srebrenica.
Mr Karadzic is alleged to have orchestrated the shelling of Sarajevo, and the use of 284 UN peacekeepers as human shields in May and June 1995.
In the Srebrenica enclave, Bosnian Serb forces overran the UN-defended safe area in the worst atrocity in Europe since the end of World War Two.
More than 7,500 Muslim men and boys were killed.
Ratko Mladic was the general in charge of the troops.
His trial is being conducted simultaneously at the International Criminal Tribunal for the former Yugoslavia (ICTY).
Mr Karadzic was arrested in Belgrade in 2008 after 13 years on the run.
He had been found living in disguise in Belgrade, under a false name and working as a New Age healer.
Mr Mladic was on the run for 16 years before being arrested in 2011 in northern Serbia, where he had also been living under an assumed name.
When Bosnia-Hercegovina became an independent state in 1992, Mr Karadzic declared an independent Serbian Republic of Bosnia and Hercegovina (renamed Republika Srpska) and himself head of state.
His party organised Serbs to fight against Bosniaks and Croats.
The Bosnian Serbs besieged Sarajevo for 44 months, shelling Muslim forces but also terrorising the civilian population with bombardments and sniper fire. Thousands of civilians died.
Bosnian Serb forces - assisted by paramilitaries from Serbia proper - also expelled hundreds of thousands of Muslims and Croats from their homes in a brutal campaign of ethnic cleansing.