Warren Buffett has donated another $1.93 billion to five charitable foundations, the third-highest amount since the investor began donating 99 percent of his wealth in 2006.

Buffett donated on Thursday 24.54 million Class B shares of his insurance and investment company Berkshire Hathaway Inc, according to a filing on Friday with the U.S. Securities and Exchange Commission.

The donation grew from $1.51 billion in 2009 because the Class B shares had risen 35 percent in price in the year ended on Thursday, when they closed at $78.81, Reuters data show.

Berkshire has benefited from an improved economy, a quiet year for catastrophe losses in its insurance units, lower paper losses on its derivatives contracts and higher prices for its common stock holdings such as American Express Co and Wells Fargo & Co.

About $1.6 billion of Berkshire shares this year went to the Bill & Melinda Gates Foundation, created by the co-founder of Microsoft Corp and his wife to address education, health and poverty problems.

Other shares went to the Susan Thompson Buffett Foundation, named for Buffett's late wife, and foundations for their children Howard, Peter and Susan. Buffett remarried in 2006.

The Gates Foundation said it employs 830 people, spent $3 billion last year, and has a $35.2 billion endowment. Buffett expects the foundation to spend his gifts, which have totaled about $8.03 billion.

Following this year's donations, Buffett still held about $43.9 billion of Berkshire stock, or 23.3 percent of the total outstanding, Friday's filing shows.

Buffett's donations totaled $1.93 billion in 2006, $2.13 billion 2007, $2.17 billion in 2008 and $1.51 billion in 2009, according to regulatory filings and Berkshire's closing stock prices on the dates the donations were made. The size of the 2010 donation is just above the 2006 donation on that basis.

The number of shares that Buffett donates declines by 5 percent each year, after accounting for a 50-for-1 stock split in January.

Last month, Gates and Buffett called on American billionaires to pledge at least half their wealth to charity. Forbes magazine in March called them the world's second- and third-richest people, trailing Mexico's Carlos Slim Helu.

Berkshire's Class A shares closed down $2,595 at $115,500 and its Class B shares were down $1.57 at $77.24.