Cefazolin was more susceptible than cephaloridine and cephalothin to in vitro inactivation by coagulase-positive, penicillinase-producing strains of Staphylococcus aureus. Inactivation (which was greater with methicillin-resistant than with methicillin-sensitive strains) was demonstrated by assay of the antibiotics in broth cultures with simultaneous colony counts and by exposure of the antibiotics to penicillinase powders extracted from S. aureus. Cefazolin was destroyed to a greater extent than was cephaloridine, whereas cephalothin underwent little, if any, destruction. The clinical implications of this degradation, thought by some to be of importance for cephaloridine, might also apply to cefazolin.