The preparation and characterization of the Fab and F(ab')2 fragments of a murine monoclonal antibody specific for aromatase cytochrome P-450 and which is suppressive of estrogen biosynthesis are described. This monoclonal antibody, MAb3-2C2, was purified from murine ascites using protein A affinity chromatography and digested with immobilized papain to produce antibody fragments. The Fab and F(ab')2 fragments were then purified using protein A affinity chromatography and S-200 HR size exclusion chromatography. The Fab fragment was further purified using S-100 HR size exclusion chromatography. Both the Fab and F(ab')2 fragments of the MAb3-2C2 suppressed aromatase activity in a dose-dependent manner. While the F(ab')2 fragment (110 kDa) maintained potent suppressive activity, the Fab fragment (42 kDa) required a higher concentration to suppress aromatase activity as compared to the IgG.