Floyd Mayweather has accused Manny Pacquiao of taking performance-enhancing drugs.
Earlier this year, Mayweather was widely expected to seal a big-money bout with Pacquiao for this month.
The Filipino instead agreed to defend his WBO welterweight title against Timothy Bradley in Las Vegas in June, and recently revealed that he wants all pay-per-view revenue to be split right down the middle before a future date with Mayweather can be agreed.
35-year-old Mayweather is currently preparing for Saturday night's Vegas clash with Miguel Cotto, but talk continues of a possible showdown between the two boxers who are widely seen as the best pound-for-pound fighters in the world.
However, 'Money' has yet again cast doubt over the possibility of the fight ever taking place by launching yet another verbal attack on his rival.
33-year-old Pacquiao has denied taking drugs and is already suing Mayweather for defamation after similar accusations were made in 2009.
But Mayweather said: "I’m going up in weight but I’m not just walking through no damn fighters.
"This motherf***er was 106lbs [when he started] and he’s just walking through Cotto. And Cotto can’t knock down [Shane] Mosley, but he can? Come on man.
"Go back and look at the pictures. His head is small and then all of a sudden his head just grew? Go back and look at the pictures and tell me this man’s head didn’t get bigger. You’re going to tell me this s*** is all natural. Come on man, stop."
Mayweather was asked by reporters why a fight with Pacquiao is yet to happen, with his response again implying Pacquiao isn't clean.
"Health is more important than anything,’ he said.
"When my career is over, if I’m hurt because of something that has happened in a fight, I can’t come to you and say, 'I need [money]'.
"People say, 'We don’t give a f*** if he’s taking or not; we just want to see the fight. We don’t give a f*** about your health and we don’t give a f*** about your family.'
"I care about my family. I love my family. They’re going to be there when no one else is there. When my career is over, you’re all going to move on to the next one."
After he faces Cotto, Mayweather will serve a 90 day jail sentence for attacking his ex-girlfriend. A fight with Pacquiao could still happen towards the end of the year and Mayweather plans to keep in shape.
"I want to stay active, I want to fight again in 2012 before the year is up," he added.
"I want to constantly go out there and test my skills and I want to continue to push myself to the limit. I still plan to retire at 37 - but I want to fight plenty of times before then."