Lawyer Rusty Hardin called McNamee's evidence a "mixed up hodgepodge of garbage" during Tuesday's opening statement.
While Hardin said the defense won't contest that a needle with steroids that McNamee gave to authorities contains Clemens' DNA, he will argue that McNamee put the steroids in the needle after injecting Clemens, and that the needle had been used to inject Clemens with vitamin B12.
Clemens is accused of lying to Congress when he denied using those drugs.
Hardin told the jurors that prosecutors won't be able to corroborate McNamee's claims, and tried to question McNamee's motives. He showed the jury the cover of an unpublished manuscript called, "Death, Taxes and MAC," with the "X" in "Taxes" made up of two crossed syringes.
"Is there any market for this book if he hadn't made these allegations about Roger Clemens?" Hardin said.
Prosecutor Steven Durham in his opening statement on Monday portrayed Clemens as a man who told lies and "other lies to cover up lies."