Cefazolin, a new semisynthetic cephalosporin with a broad spectrum of antibiotic activity, was studied in a series of toxicologic experiments. It had a very low acute toxicity to mice and rats when administered both parenterally and orally. Cefazolin was not teratogenic for mice or rabbits and had no effect on the reproductive performance or on the offspring of rats treated prior to and during mating, gestation, and lactation. In experiments of subacute and chronic toxicity in rats and dogs, the major finding was damage to muscles following im injection. In studies of nephrotoxicity in rabbits, iv cefazolin, at doses of 300 mgtkg, produced degenerative changes in the kidney. Cephaloridine, used for comparative purposes, produced tubular necrosis of the renal cortex at one-third this dose, i.e., 100 mg/kg.