The mammary glands of virgin, pregnant, and lactating BALB/c mice, the preneoplastic hyperplastic alveolar-nodule outgrowths, and mammary adenocarcinomas from BALB/c mice were examined for glucose-6-phosphate dehydrogenase (G6PD) isoenzymes. Normal mammary gland contained 2 isoenzymes (G6PD-2, G6PD-3), whereas a third, more rapidly migrating isoenzyme (G6PD-1) was found in significant levels only in the preneoplastic and neoplastic mammary tissues. Thus expression of a specific G6PD isoenzyme seems related to neoplastic transformation.