Warren Buffett's annual gift of Berkshire Hathaway stock to the Bill and Melinda Gates Foundation is worth $2.1 billion this year, up from $2.0 billion last year. It's also more than any of Buffett's previous eight donations.

Even though Buffett's gift schedule called for him to give the Gates Foundation 16.6 million Class B shares this year, down from 17.5 million shares last year, the value of each share gained 12.1 percent over the past 12 months, boosting the total value of the donation by more than $100 million above last year's level.

It's the third-consecutive year the value of Buffett's gift has increased from the previous year, even though he donated fewer shares each time.

In a 2006 letter to Microsoft's co-founder and his wife, Buffett outlined a long-term schedule that has him donating almost all of his fortune by giving 5 percent fewer shares each July, when compared to the previous year.

Since he began giving to the Gates Foundation in 2006, Buffett has handed over nearly 185 million shares with a total market value of $15.1 billion as of their donation dates.

At Monday's close of $128.98, that stock would be worth $23.8 billion.

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In an SEC filing, Buffett also discloses a donation of $214 million to the foundation named for Susan, his first wife, and donations worth almost $150 million to each of the foundations run by his three children.

After converting 15,000 of his Class A shares into Class B to make donations, Buffett now holds 321,000 Class A shares, just over 38 percent of the shares outstanding.

—By CNBC's Alex Crippen.