Four mouse monoclonal antibodies reacting with renal proximal epithelium were characterized on normal adult tissues, fetal kidneys, and various malignancies. These four antibodies were found to react with renal cell carcinomas (RCCs) but not with a variety of other tumors. Three antibodies react only with some primary RCCs and not with metastatic RCCs. The fourth antibody, RC 38, reacts with 46 of 47 primary RCCs tested and with 8 of 13 RCC metastases. No reaction was seen with 179 tumors of various origin, which indicates that this antibody is very useful in diagnosing RCC.