Several investigators have shown that changes occur in isoenzyme activities of lactate dehydrogenase (LDH) and glucose-6-phosphate dehydrogenase (G-6-PD) as tissues become neoplastic [5, 7, 10]. Acrylamide-gel electrophoretic studies of two Syrian hamster tumors have demonstrated changes in the isoenzyme patterns of LDH and G-6-PD during tumor progression. A stilbestrol/testosterone-induced scent-gland tumor develops the typical LDH picture of malignant tissues (high concentration of the slowest moving isoenzyme) as it progresses to its final metastatic state [2, 3]. The G-6-PD isoenzyme pattern of a stilbestrol-induced and stilbestrol-dependent renal carcinoma differs from that of an autonomous hormone-independent variant derived from it [4]. Since the estrogen-induced tumor of the kidney of the Syrian hamster was first described [6], several hormone-independent autonomous variants and one spontaneous renal tumor have appeared in Kirkman’s hamster colony. The observations on these tumors described in this paper suggest the possibility of using (LDH) and (G-6-PD) isoenzymes analysis in distinguishing clinically between hormone-dependent and hormone-independent renal tumors.