McNamee testified this week that he injected Clemens with performance-enhancing drugs in 1998, 2000 and 2001.
Clemens, a former pitching star, is accused of lying to Congress when he denied using steroids and human growth hormone.
Clemens' lawyer has spent a little over two hours cross-examining McNamee, trying to portray him as a liar with a bad memory, and that will continue today.
McNamee, Clemens' former friend and longtime strength coach, is central to the prosecution because he's the only person who will claim firsthand knowledge of Clemens taking performance-enhancing drugs.
Clemens spent 24 seasons in the major leagues, pitching for Toronto, Boston the New York Yankees and Houston.