To assess the possibility that non-aminoglycoside antibiotics may adversely affect the nephrotoxicity of the new semisynthetic aminoglycoside netilmicin, we gave ampicillin, carbenicillin, methicillin, cefamandole, and clindamycin, either singly or in combination with netilmicin, at two dose concentrations in rats. Results were compared as to the effect of netilmicin given singly and to saline-injected and noninjected controls. Antibiotic combinations resulted in no greater nephrotoxicity than did netilmicin alone. Netilmicin concentrations in renal tissue were high, and these levels were not consistently affected by the other drugs. The data suggest that in rats the nephrotoxicity of netilmicin is not affected adversely by the presence of non-aminoglycoside antibiotics.