In recent years there has been a gradual increase in the number of infections caused by Klebsiella, Enterobacter and Serratia and non-fermentative bacilli such as Pseudomonas, Alcaligenes and Acinetobacter. The object of this investigation was to evaluate the antibacterial properties of the cephalosporins cefoxitin, cefoperazone, cefotaxime and of gentamicin, in comparison with cefotetan, a new cephamycin with a high degree of stability to beta-lactamases. The cefotetan MIC was not affected either by size of the bacterial inoculum, or the culture medium and the serum protein binding of cefotetan was 85%. Cefotetan showed only moderate antibacterial activity, similar to that of the other cephalosporins tested, against the Gram-positive cocci, whereas it was very active against fermentative and non-fermentative Gram-negative bacilli, except for Pseudomonas aeruginosa. The remarkable activity of cefotetan against Klebsiella, Enterobacter and Serratia is of particular interest.