Cefoxitin is a new β-lactam antibiotic of the cephamycin class. It has been extensively studied for adverse reactions in animals, volunteers, and patients. The overall human experience is based on data obtained in 1749 subjects. A large proportion of these were patients with serious or even life-threatening infections, often complicated by one or more underlying diseases. Hypersensitivity reactions were observed in a frequency similar to that found with the cephalosporins. No case of anaphylactic shock has been reported. In contrast to some of the cephalosporins, cefoxitin seemed not to cause any impairment of renal function, nor did concomitant treatment with gentamicin or furosemide (frusemide) precipitate nephrotoxic reactions. The few gastrointestinal disturbances reported, indicated that cefoxitin does not significantly affect the normal bowel flora. In a small percentage of patients, reversible, mild or moderate, increases of liver transaminases and/or alkaline phosphatases were noted. Positive direct Coombs test without haemolysis was found in a frequency similar to that found with the cephalosporins. Clinical superinfections caused by cefoxitin-resistant microorganisms, mainly Pseudomonas, Enterobacter and fungi, were observed to the same extent as can be expected with other broad spectrum antibiotics. The available experience clearly demonstrates that cefoxitin shares the safety of the other β-lactam antibiotics without causing the high frequency of rashes observed with ampicillin and is free of serious nephrotoxicity. © 1979 The British Society for the Study of Infection.