Biochemical parameters characterizing growth, functional activity, and hormone dependence were compared in the normal mammary gland, the 7,12-dimethylbenz(a)anthracene-induced hyperplastic aveolar nodule (HAN), and the mammary tumor in the rat. The rate of DNA synthesis in HAN was significantly lower than the rates in either the normal mammary gland cells or the mammary tumor cells cultured in medium containing identical hormone supplement for the same duration of time. The rate of casein synthesis in the explants of pregnant rat mammary gland and HAN was greatly stimulated when cultured in medium containing insulin, cortisol, and prolactin. The mammary tumor, however, failed to produce an increase in casein synthesis under the same experimental conditions. The specific estradiol-binding proteins were either absent or present in very low concentrations in HAN. In contrast, mammary tumor cytosol contained high concentrations of specific estrogen-binding proteins. The normal mammary gland, in spite of its low cellularity, had twice as many estradiol-binding macromolecules as did HAN. Altogether, these results show that HAN cell populations differ from normal mammary gland and mammary tumor cells by their nonresponsiveness to hormonal stimulation for growth and by their lack of specific receptors for extradiol. They retain, however, the functional capacity to synthesize casein, a biochemical property that the mammary tumor does not posses.