The susceptibility of 100 group B streptococci to 16 ,-lactam antibiotics was tested by agar dilution. Penicillin G and N-formimidoyl thienamycin were the most active agents tested, both having a 90% minimal inhibitory concentration (MIC90) of 0.06 ,ug/ml. Ceftriaxone, cefotaxime, cefamandole, and SCH 29482 were almost as active, all having an MIC90 of 0.12 ,ug/ml, and ampicillin, cephalothin, and mezlocillin all had an MIC%0 of 0.25 ,ug/ml. The MIC90 for piperacillin, cefoperazone, and ceftazidime was 0.5 jig/ml. Least (Schering Corp.). These agents were supplied as laboratory standard powders of known potency, and stock solutions were made as recommended by the manufacturers.MICs were determined by the agar incorporation method in plastic petri dishes (15 by 100 mm) containing 25 ml of Mueller-Hinton agar supplemented with 5% sheep blood and incorporating the above antimicrobial agents in concentrations of 0.008 to 256 ,ug/ml in doubling dilutions (13). After pouring, the plates were dried, wrapped in plastic, and refrigerated at 4°C before use within 7 days.For MIC testing, isolates were grown in 2 ml of Mueller-Hinton broth at 35°C for 3 to 5 h, and the turbidity was then adjusted to a turbidity equivalent to that of a 0.5 McFarland barium sulfate standard with a nephelometer (1R, API; Analytab Products). Suspensions were further diluted 1:20 to obtain a final inoculum of 104 organisms per 2 ,u. Plates were inoculated with a Steers replicator with 3-mm inoculating pins and inoculated overnight at 35°C in room air. The MIC was defined as the lowest concentration of antibiotic completely inhibiting the growth of each isolate. Quality control was ensured if results obtained with recommended controls (13)