Google pays Apple Inc. 36% of the revenue it earns from search advertising made through the Safari browser, the main economics expert for the Alphabet Inc. unit said Monday.

Kevin Murphy, a University of Chicago professor, disclosed the number during his testimony in Google’s defense at the Justice Department’s antitrust trial in Washington.

John Schmidtlein, Google’s main litigator, visibly cringed when Murphy said the number, which was supposed to remain confidential.

Both Google and Apple had objected to revealing details publicly about their agreement. In a court filing last week, Google argued that revealing additional information about the deal “would unreasonably undermine Google’s competitive standing in relation to both competitors and other counterparties.”

The companies have had a partnership since 2002 that makes Google the default search engine in Apple’s Safari. Today that deal is the most important of Google’s default deals, since it sets the search engine for the iPhone, the most used smartphone in the US.

The Justice Department is targeting that agreement as evident Google is illegally maintaining its dominance over the search engine and search advertising markets.

Apple didn’t immediately respond to a request for comment. Google declined to comment.

(Updates with additional details beginning in fifth paragraph.)