Jones, who was set to fight Russian Denis Lebedev (25-2, 19) for the WBA Cruiserweight title, has allegedly tested positive for a banned diuretic. Unfortunately for the 6'4" Cruiserweight this isn't the first time and he actually tested positive for a similar type of drug last year, when he beat Lebedev in very controversial fashion, and way back in 2002 when he fought to a draw with Johnny Nelson. Although he tested positive in both of those bouts his record still counts the win and the draw with neither bout over-turned to a no-contest.
We are awaiting confirmation of this story and what will happen next but it seems likely that the fight will either be off or Jones will be unable to win the title and will face a ban pending a hearing after the bout. It's a real shame for Lebedev if the bout is cancelled and robs him of his chance to avenge his "loss" from last year though the test could explain why Jones, in his 40's, can still make Cruiserweight despite being a huge man for the division.
From what we understand the fight will go on, with the likely out come to be a non-title fight.
EDIT 2: The WBA have CONFIRMED the story! The fight might go ahead but the WBA will have no part in it.