Cefotaxime, a new semisynthetic cephalosporin derivative, showed a broad spectrum of antibacterial activity against clinically isolated strains of gram-positive and gram-negative bacteria. This cephalosporin was slightly less active than cefazolin against Staphylococcus aureus but 4 to 300 times as active as carbenicillin against gram-negative organisms, including Pseudomonas aeruginosa, Pseudomonas cepacia, Enterobacter cloacae, and Serratia marcescens. Cefotaxime was the most active compound against members of the Enterobacteriaceae and 20- to 100-fold more active than cefoxitin against the indole-positive Proteus group. The minimal bactericidal concentrations of the compound were identical to, or two times higher than, the minimal inhibitory concentrations against Escherichia coli and P. aeruginosa and four times higher against S. marcescens. A reduction of inoculum size decreased greatly the minimal inhibitory and bactericidal concentrations of cefotaxime against E. coli P. aeruginosa, and S. marcescens. The antibiotic was very stable to penicillinase and cephalosporinase produced by gram-negative bacteria, including Proteus vulgaris.