Jack Dorsey, the CEO of Twitter, said Tuesday he will donate $1 billion worth of equity in Square to his Start Small Foundation to fund COVID-19 relief around the world. 

Dorsey made the announcement in a tweet, revealing that the sum equates to roughly 28 percent of his current net worth, or about $3.6 billion. The announcement marks the most significant philanthropic effort from the 43-year-old tech executive in his career.

According to a Google Doc that Dorsey set up to disclose his donations, the first funding he contributed was $100,000 on April 2 to America’s Food Fund, an initiative launched by Leonardo DiCaprio, Apple, Laurene Powell Jobs, and the Ford Foundation and backed by Oprah Winfrey to provide food to those in need during the coronavirus crisis.