The antibacterial activity of cefoxitin and cephalothin was tested against 7,312 clinical isolates of aerobic pathogens. Cefoxitin was found to have a broad spectrum of activity. Among the gram-positive cocci, 99.1% of Staphylococcus aureus, 94.4% of Staphylococcus epidermidis and 98.1% of streptococci group B had a minimal inhibitory concentration of 16 microgram/ml or less, while only 4.6% of enterococci were sensitive to this concentration of cefoxitin. Among the gram-negative rods 94.4% of Escherichia coli, 89.3% of Klebsiella spp., 97.3% of Proteus mirabilis, 97.4% of Proteus vulgaris and 79.3% of Proteus morganii were sensitive to 16 microgram/ml of cefoxitin or less. The effect of Pseudomonas spp. and Acinetobacter calcoaceticus was negligible. Cefoxitin was less active than cephalothin against gram-positive cocci, the difference being most pronounced against enterococci. Cephalothin was also found to be more active than cefoxitin against P. mirabilis, while cefoxitin was superior to cephalothin against E. coli, Klebsiella spp., Enterobacter spp., P. vulgaris and P. morganii.