THE ORIGINAL INVESTIGATIONS of the new penicillinase-resistant penicillins demonstrated that their antimicrobial activity could be destroyed by sufficiently large concentrations of Bacillus cereus penicillinase.1 During the course of experiments dealing with the bactericidal activity of such penicillins, it was essential to antagonize residual antibiotic in subcultures to avoid false-negatives. For this to be accomplished, the actual concentration of penicillinase necessary for this purpose had to be established. This information also could be extended to clinical laboratories where blood cultures are made from patients under treatment with these drugs. We have therefore investigated this problem via a study of ratios of B cereus penicillinase to penicillin required for inactivation of four penicillinase-resistant penicillins. For practical laboratory applications it is probably fortunate that B cereus penicillinase, which is easily obtainable, is more potent than staphylococcal penicillinase against resistant penicillins.1 It is the purpose of this paper to report these studies.