The minimum inhibitory concentrations of temocillin against more than 1000 clinical isolates were determined by an agar dilution method. Temocillin showed excellent activity against Haemophilus influenzae, pathogenic Neisseria species and Branhamella catarrhalis, including beta-lactamase producing strains, but showed very low activity or was inactive against Gram-positive cocci and Campylobacter, Bacteroides, Acinetobacter and Pseudomonas species. Good activity was obtained against 702 Enterobacteriaceae, including isolates resistant to the other penicillins and first- and second-generation cephalosporins, with 92% of all the strains inhibited at a concentration of 8 mg/L. However, the most striking property of temocillin was its high beta-lactamase stability which resulted in both a very narrow range of MICs within which all the isolates were inhibited, and a small influence of inoculum size on the MICs.