Actor Will Smith met Nelson Mandela on Thursday after agreeing to become an ambassador for his Aids campaign.
Nelson Mandela said Smith had "power" over his young fans
The star met Mr Mandela in South Africa after the country's 86-year-old former president offered him the role to help educate young people around the world.
Smith said he would "humbly, gratefully
and aggressively" answer the call.
Beyonce Knowles and Missy Elliot have also donated proceeds from their concerts to the campaign, named 46664 after Mr Mandela's old prison number.
Smith, who is promoting his latest film I, Robot in South Africa, said he first talked about the issue with Mr Mandela during a previous visit to the country four years ago.
"I was saying to Mr Mandela 'I am an actor, I make rap music that's what I do, what can I do?'
"He said 'you have to understand the power that you have and the hope that is created by what you do.'"
Smith said the last time he met Mr Mandela, he became "so fired up" he was ready to quit his job.
Mr Mandela said in a statement he was convinced the Men in Black star would help spread awareness of HIV/Aids to those most at risk.
"His ability to inspire us will encourage the world's young people to stay HIV negative in order to achieve more," he said.
But Mr Mandela also urged world leaders to make "personal commitments and concrete actions" in the fight against HIV/Aids.
He said: "They will be faced with tough decisions and they must come up with bold and innovative responses.
"This is what leaders are for and the Aids epidemic will test their leadership skills to the limits.
"Mr Will Smith, you have demonstrated here that you are a leader, and we are honoured to have you associated with 46664."
Other celebrity supporters of the 46664 campaign include Bono, Peter Gabriel and Sir Bob Geldof.