Roger Clemens is stepping up to the mound for Mayor Giuliani’s Twin Towers Fund.

The Yankees’ star pitcher is auctioning two custom cars he owns, a Mercedes and a Porsche.

Half of the proceeds will go to the Twin Towers Fund and the other half to the Roger Clemens Foundation’s own efforts on behalf of the victims of the World Trade Center terror attack.

The “Gotta Have It – Baseball for America” charity auction will feature more than 75 lots of sports memorabilia from the nation’s biggest baseball legends.

They include items from Babe Ruth, Lou Gehrig, Joe DiMaggio, Ty Cobb, Mickey Mantle, Roger Maris, Willie Mays, Jackie Robinson, Thurman Munson, Mark McGwire and Barry Bonds.

The auction is being held online at sothebys.com and gottahaveit.com.