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Cysteine conjugate beta-lyase, an enzyme that converts cysteine S-conjugates to free thiols, pyruvate and ammonia, is normally expressed primarily in the liver and kidney. In theory, this selective distribution affords the opportunity to target thiol-containing drugs to these organs and, perhaps, to tumors derived from them. To assess the potential for delivery of such drugs to kidney-derived tissue, we have used a typical beta-lyase substrate, S-(2-benzothiazolyl)-L-cysteine, to measure the beta-lyase activity in normal and tumor tissue of kidneys removed from patients with renal carcinoma. Although considerable heterogeneity in enzyme activity levels was observed in normal and tumor-derived samples, a high proportion of tumor samples had enzyme activity that was at least 50% of that observed in adjacent normal tissue. Frequently, hypoxanthine-guanine phosphoribosyltransferase activity was observed to be greater in the tumor than in normal tissue. These results may aid in the development of therapy for renal carcinomas.