Sera from seven animal species (rat, cow, cat, dog, human, rabbit, and hamster) were tested and found to inhibit the papain-catalyzed hydrolysis of alpha-N-benzoyl-L-arginine-p-nitroaniline-HCl (L-BAPA). The relative concentration of inhibitor in each serum sample was expressed in terms of its papain inhibitory capacity (PIC) defined as the number of units of papain inhibited per ml of serum. Rat serum contained the highest concentration of inhibitor while hamster serum contained the lowest concentration. The inhibitor appears to be competitive with respect to L-BAPA and is heat labile and nondialyzable. Antipapain activity can be separated from antitrypsin activity by (NH4)2SO4 fractionation.