The cephalosporin antibiotics cephaloridine and cephalothin are known to cause renal damage. Experience with newer congeners is not yet sufficient to predict their potential nephrotoxocity. The renal lesion produced by cephaloridine is primarily due to the intrinsic toxicity of this drug for the cells of the proximal renal tubule and depends upon its peculiar transport characteristics. In contrast, renal injury due to cephalothin resembles that seen with the penicillins. Thus, some instances of cephalothin nephropathy appear to be toxic in nature with a histologic picture of acute tubular necrosis, whereas others exhibit signs of hypersensitivity including rash, eosinophilia, and interstitial nephritis. Among the factors alleged to contribute to the nephrotoxicity of cephalosporins is their administration with aminoglycosides. Although the physician should be aware of the possibility of a potential adverse interaction between these groups of antibiotics, the evidence is not sufficiently conclusive to warrant avoidance of the combination when it appears to be therapeutically useful.