Isozyme profiles for 32 enzyme systems were studied in tumors induced by two strains of polyoma virus (2PTA and LID1), in two conventional mouse strains (C3H/BiDa and NIH), and in athymic (nude) mice of two genetic backgrounds (C3H/Hes nu/nu and NIH nu/nu). Tumors studied were: primary and transplant passages of salivary gland tumors (127); primary thymic epithelial tumors (12); primary subcutaneous sarcomas (6); primary hair follicle tumors (5); primary and transplant passages of mammary tumors (18); primary ameloblastomas (3); and primary renal medullary sarcomas (3). Regardless of mouse strain or virus strain, the isozyme arrays were highly constant and unique for each tumor histotype with the exception of salivary and mammary tumors, which shared a single profile differing from that of each of the other histotype-associated profiles. Other tumor types could be distinguished from each other and from the salivary-mammary tumor pair by as few as five isozymes: glycerol-3-phosphate dehydrogenase; glyceraldehydephosphate dehydrogenase; lactate dehydrogenase; sorbitol dehydrogenase; and alkaline phosphatase. Twelve nonpolyoma mammary tumors and their passages from mouse mammary tumor virus-expressed C3H/Hes nu/+ mice were analyzed for the same enzymes; variations in activity and isozyme profiles were found for ten enzyme systems. Three spontaneous salivary myoepitheliomas in BALB/c mice were also analyzed; two different lactate dehydrogenase profiles were observed, and all three tumors lacked the placental alkaline phosphatase present in polyoma virus-induced salivary tumors. Uniformity of isozyme phenotype may be characteristic of DNA virus transformation of cells in a particular differentiative state. This uniformity does not appear to occur in mouse mammary tumor virus-associated tumors, spontaneous tumors, and, according to the literature, chemically induced tumors.