Forty of the richest families and individuals in the United States, including at least 30 billionaires, have pledged to give at least half of their wealth to charity.

They announced their intentions Wednesday as part of a campaign started by famed investor Warren Buffett and Microsoft founder Bill Gates.

Buffett and Gates launched the "Giving Pledge" campaign in June, trying to convince the wealthiest of Americans to give their fortunes to worthy causes during their lifetime or after they die.

Buffet called the 40 pledges a terrific start.  

Those making the commitment includes some of the country's best-known billionaires, including New York City Mayor Michael Bloomberg, media mogul Ted Turner, oil tycoon T. Boone Pickens and "Star Wars" movie director George Lucas.

Buffett and Gates say the "Giving Pledge" campaign is meant to inspire charitable giving and does not collect money for any specific charities.

Earlier this year, the U.S. magazine Forbes ranked Gates as the world's second richest person with an estimated fortune of $53 billion.  Buffett was ranked third with $47 billion.

In 2006, Buffett pledged to donate most of his wealth to Gates' foundation and four other charities.

The Bill and Melinda Gates Foundation raises billions of dollars to fight poverty and disease around the world.