Buffett said he, Gates and his wife and others have made 70 to 80 calls to some of America's wealthiest individuals.
Among those who have not signed the pledge, were some who prefer to keep their philanthropy anonymous, some who were not available to talk, and others who were not interested, Buffett said.
Many on the list will be asked to call others, and small dinners will be held across the country in coming months to talk about the campaign.
"We're off to a terrific start," Buffett said.
Buffett said he and Gates will meet with groups of wealthy people in China and India within the next six months to talk about philanthropy.
They hope the idea of generosity will spread, but they have no plans to lead a global campaign, Buffett said.
Buffett decided in 2006 to give 99 per cent of his fortune to charity. Then, he was worth about $44bn.
Bill and his wife Melinda do most of their philanthropic giving through their foundation, which had assets of $33bn as of June 30 and has made at least $22.93bn in total grant commitments since 1994.
Gates and Buffett are asking billionaires not just to make a donation commitment, but to also pledge to give wisely and learn from their peers.
No combined charity
Their group has no plans for combined giving, and none of the philanthropists will be told how or when to give their money.
"Everybody has their own interests," Bloomberg, who participated in a teleconference with Buffett on Wednesday, said.
In 2009, American philanthropies received a total of about $300bn in donations, according to The Chronicle of Philanthropy.
Stacy Palmer, editor of The Chronicle, was surprised and impressed by the speed at which the giving pledge idea has been accepted.
"I think it's remarkably fast that so many people went public with their commitments," she said.
"The world of philanthropy tends to be very slow moving."
Palmer noted that many of the names on the list are people who are known for their philanthropic generosity.
She said she would be more excited when she sees names that have not been on other major donor lists.
Taking the idea past billionaires towards millionaires and regular working people could make an even bigger impact, Palmer added.
Jason Franklin, executive director of Bolder Giving, a relatively new organisation that encourages big gifts from everyday people, agreed, saying he estimates the giving power of the world's millionaires eclipses the potential donations from US billionaires many times over.