Determination of the kinetic constants Km and Vm showed little or no interaction between ceftazidime and β -lactamases of the penicillinase type, such as TEM-1. Some interaction was observed with β -lactamases of the cephalosporinase type isolated from Escherichia coli, Enterobacter cloacae, Proteus morganii and Pseudomonas aeruginosa , but the geometric mean of the inhibition constant Ki used to measure this interaction was 120 and almost 700 times higher with ceftazidime than with cefotaxime and cefuroxime, respectively. Two other β -lactamases isolated from Pr. vulgaris and Klebsiella oxytoca also showed little or no interaction with ceftazidime. A complementary microbiological technique, the “double-disc” technique, indicated that ceftazidime was more stable than cefotaxime to all β -lactamases tested. Moxalactam, however, appeared to have slightly greater stability.