Metastatic renal cell carcinoma (RCC) is a tumor with a poor prognosis resistant to chemotherapy and irradiation. However, in rare cases, spontaneous regressions of metastases after nephrectomy have been reported that were ascribed to destruction of the tumor by the immune system. In earlier studies, we and others identified the expression of modified antigens in RCC. In particular, a concanavalin A-binding isoform of γ-glutamyltrans-ferase (γGT) comprises about 50% of the enzymatic activity found in the tumor tissue of many RCC patients. The monoclonal antibody 138H11, directed against all human γGT isoforms, revealed a membranous expression of γGT around RCC cells compared to the restricted, apical pattern of renal cells. These data raised the question why the RCC-specific γGT, exposed to the immune system in vivo, is not immunogenic in patients with RCC. To address this question, we generated mouse hybridomas against highly purified human RCC γGT. Although we obtained a large panel of hybridomas that produced antibodies reacting immunohisto-chemically in a γGT-specific manner, all antibodies stained normal kidney γGT as well as RCC γGT. These results suggest that the biochemical features specific for RCC γGT are not necessarily reflected by specific anti-genic determinants that could be differentiated by the immune system of the challenged mice. The lack of immunogenic epitopes different from normal γGT enzyme may offer an explanation why γGT produced by RCC can escape immunological surveillance.