Colombia's government has offered to free jailed rebels in exchange for hostages held by the left-wing Farc group - a reversal of earlier policy.
Families of the rebels' hostages have campaigned for their release
The offer was made to the Farc in July, but has only just been made public.
Until now, President Alvaro Uribe's government has refused to free captured rebels in exchange for the Farc's many high-profile hostages.
The government now says it will trade 50 rebels provided they do not again take up arms against the state.
"There are two possibilities," government negotiator Luis Carlos Restrepo said. "They [the freed rebels] can leave the country or take up the government's project to re-integrate them into society."
Farc guerrillas have held hundreds of people hostage in secret jungle camps, many for several years.
Their captives include US military contractors, former presidential candidate Ingrid Betancourt and scores of Colombians held for ransom money.
The Farc rebels want thousands of their jailed comrades freed
Hostages' families - and perhaps, the US government - have been pushing Bogota to alleviate their plight, says the BBC's Jeremy McDermott in Bogota.
He says huge hurdles remain before any exchange will be possible because of the conditions the government has set for releasing jailed rebels.
A spokesman for President Uribe said the Farc had yet to respond to the government's offer, which he described as "a major policy change".
In the past, the rebels have said they want the government to release thousands of their jailed colleagues and allow them to rejoin their violent struggle for a Marxist state.
Hundreds of thousands of people have died in the 40 years since the Farc guerrillas began their fight.