Cefmenoxime was evaluated in an open trial consisting of 41 patients. Forty infections in 36 patients could be evaluated. Thirteen patients had pyelonephritis due to Escherichia coli (two bacteremic), Pseudomonas aeruginosa, Klebsiella pneumoniae, or Streptococcus faecalis; all improved and 12 of 13 were clinically cured, but one relapse (S. faecalis) occurred at two weeks. Six patients with cystitis due to E. coli, Citrobacter freundii, Serratia marcescens, P. aeruginosa, or S. faecalis all improved, but relapse or reinfection, or both, occurred in five due to P. aeruginosa, S. faecalis, C. fruendii, or E. coli. Neurogenic bladder or other complications were present in five of 13 patients with pyelonephritis and five of six with cystitis. Ten patients with pneumonia and one with tracheobronchitis due to Hemophilus influenzae, S. pneumoniae, S. agalactiae, or Neisseria meningitidis all improved and seven had resolution without relapse, but P. aeruginosa emerged in two patients, one of whom died. Eight soft tissue infections due to Staphylococcus aureus, Peptococcus prevotti, Streptococcus species, or infections of mixed origin resolved in six. Sterility of blood cultures was obtained in one patient with endocarditis due to S. anginosus, but other therapy was substituted. Clinical resolution of the toxic shock syndrome and subsequent negative endocervical cultures for S. aureus occurred in one. Granulocytopenia of unverified cause in four (with less than 1,500 mm3) and two (with less than 2,000 mm3) was reversible. Headache during treatment occurred in six patients and a possible disulfiram-like effect in three. Elevations of serum glutamic oxalacetic transaminase and alkaline phosphatase occurred in five, Coombs' positivity in two, and diarrhea in three. Clinical efficacy of cefmenoxime was significant. Possible side effects require further study.