We report a novel way of obtaining a monoclonal antibody to renal cell carcinoma (RCC). BALB/c mice were immunized with RCC cells (ACHN, ATCC CRL1611) and hyperimmunized spleen cells were fused with Sp/2 murine myeloma cell line by PEG 2000. Many hybridoma supernatants were screened by enzyme linked immunosorbent assay (ELISA). After the cloning by limiting dilution, we established a hybridoma reactive to RCCs and named it A25. It belonged to the IgG1 subclass of immunoglobulins. According to our results using ELISA, 4 of 5 RCC cell lines were reactive to A25, while the remaining 23 non RCC cell lines did not react. The supernatant from A25 was used as a primary antibody preparation for avidin-biotin complex immuno peroxidase staining of multiple cases of RCC, normal tissue, and other tumors. This antibody reacted with 3 of 3 grade 1 RCC, 10 of 11 grade 2 RCC and 0 of 3 grade 3 RCC. Proximal tubules of the kidney shared this antigen. However, cross reactivity of this antibody was observed to pyloric glands of the stomach and adenocarcinoma of the colon. The epitope of A25 seemed to originate from normal kidney tubules. Low grade tumors preserved this epitope well, but this character of the original tissue seemed to disappear as tumor grade increased.