Cefotaxime was used to treat 33 patients with serious Gram-negative bacillary infections. The dosage varied between 1.5 g and 4 g/day in patients without renal failure. Eighteen patients had urinary tract infections, 6 had pulmonary infections and 10 had miscellaneous infections. Sixteen patients were also bacteremic. Twenty-five infections were due to pathogens resistant to ampicillin, cephalothin, gentamicin and/or tobramycin, in vitro. Fifteen infections had failed to resolve during ampicillin, cefazolin, gentamicin or tobramycin therapy. Thirty-two patients responded favourably (cure or improvement). Four patients developed superinfection with cefotaxime-resistant bacteria. One patient developed mild reversible renal insufficiency. Cefotaxime is a very active cephalosporin with potential use in serious multi-resistant enterobacteria infections