Undefeated boxer Floyd Mayweather and mixed martial arts champion Conor McGregor traded barbs during a lively press conference in Los Angeles on Tuesday, delighting a rowdy crowd ahead of their Aug. 26 fight in Las Vegas.

The media conference was the first in a four-day international tour promoting the bout.

The contrasts between the two was immediately apparent as McGregor, wearing a navy pinstriped suit and tie, danced around the stage smiling while Mayweather, wearing a track suit with American flag details, looked on stonefaced.

As is expected in the bout, McGregor was the aggressor, wasting no time bringing up reports that Mayweather earlier this month asked the IRS for more time to pay his 2015 tax bill.

"He's in a track suit," McGregor told the crowd of more than 11,000 people, who largely cheered the Irishman and booed the American. "He can't even afford a suit anymore."

Later when Mayweather, whose nickname is "Money" and who frequently uses cash as a prop, pulled out a $100 million cheque on stage, McGregor told him to "give it to the taxman".

When asked about the tax issue, Mayweather said his attorneys were working on it and did not worry him.

"I just showed you all a $100 million cheque that I haven't even cashed yet, so we ain't tripping on that," he told reporters.

The cheque appeared to be the $100 million he earned for his victory over Manny Pacquiao two years ago.

Mayweather was more subdued than the fiery 28-year-old, reminding reporters that even though he is a heavy favourite, he is 40 years old and that anything can happen in a fight.

He said he expected McGregor, who has never fought a professional boxing match, to try to pummel the defensive-minded Mayweather, who is putting his perfect 49-0 record on the line.

"He's upset, I've seen it before," Mayweather said. "And when these fighters get upset with me they come out and they try to kill me," he said with a laugh.

"My job is to do what I do and keep my composure."

The crossover fight has been criticised as a publicity stunt by boxing purists who give McGregor no chance.

But UFC president Dana White said the fight is one that fans of both sports have wanted.

"When two guys get in there and start throwing punches, anything is possible," White said.

"If you are a fan of combat sports, there has never been a better time."