These studies were undertaken to determine if altered growth potential of mammary epithelial cells could be detected in outgrowths derived from monodispersed mammary cells of virgin female BALB/c mice previously exposed to ionizing radiation or 7, 12-dimethylbenz(a)anthracene (DMBA). Monodispersed mammary epithelial cells were obtained by enzymatic dissociation of mammary tissues of 12-week-old virgin female BALB/c mice. Twenty-four hr prior to cell dissociation, donor animals were exposed to either 100 rads of gamma-ray irradiation, 0.25 mg of DMBA, or 0.075 mg of DMBA. Control donors were untreated. Mammary outgrowths were then derived from these donor cells by injecting either 10(5) or 10(4) cells into the gland-free mammary fat pads of three-week-old virgin female BALB/c mice. Ten weeks after the injection of cells, the outgrowths were examined and classified. Mammary outgrowths were classified either as having a normal ductal architecture or as having ductal dysplasia. Ductal dysplasias were further classified on the basis of an index of severity, which was an arbitrary index based on the number of abnormal ductal structures within each lesion. The data indicated that treatment of donor animals with either gamma-radiation or DMBA increased the frequency of ductal lesions over control levels; however, both the frequency and severity of the lesions depended on the number of cells which were injected into the fat pad. When outgrowths were derived by injection of 10(5) cells into the gland-free fat pads, lesion frequencies in outgrowths from control and treated cells were: 3.3%, control; 15.7%, gamma-rays; 5.3%, 0.25 mg DMBA; in these groups only a few severe lesions were detected. In outgrowths derived from 10(4) cells, less severe lesions (Class I lesions) were common in all groups and occurred in approximately 10 to 15% of the outgrowths. The frequency of severe (Class II and III) ductal dysplasia, however, was increased by treatment in these groups, occurring in 4.5% of control outgrowths in 15.6, 14.9, an 14.3% of the outgrowths derived from donor cells treated with 100 rads gamma-rays, 0.075 mg DMBA, nd 0.25 mg DMBA, respectively. Thus, these data indicated that ductal dysplasias were more common and more severe in outgrowths derived from 10(4) rather than 10(5) cells. The ductal lesions observed in this study resembled both morphologically and histologically ductal abnormalities which have been associated with the pathogenesis of mammary carcinoma in both rats and mice.