Because of the potential for an interaction between cephalosporins and aminoglycosides leading to renal injury, an evaluation of the effect of a new cephalosporin, cefamandole nafate, on the toxicity of the aminoglycoside tobramycin was performed in laboratory animals. High doses of cefamandole nafate did not increase the acute toxicity (lethality) of tobramycin in rats or mice. In a subacute experiment in rats, dose-related tobramycin nephrotoxicity, as evidenced by blood urea nitrogen changes, increased kidney weights, and histologically determined tubular nephrosis and necrosis, was observed. Concomitant treatment with cefamandole nafate, 500 mg/kg, did not increase tobramycin nephrotoxicity, but protected against the aminoglycoside-induced renal injury. Determination of tissue radioactivity after administration of [(14)C]tobramycin indicated that cefamandole nafate treatment resulted in uniformly lower tobramycin tissue concentrations compared with the control, suggesting that the protective effect was produced by enhanced excretion of tobramycin after cefamandole nafate treatment.