There is only indirect evidence at present to suggest a role for PRL in either the genesis or progression of human breast cancer. Here, we report the results of experiments in primary cultures of breast tumor cells from a hyperprolactinemic breast cancer patient who had an elevated mean 24-h PRL concentration but a normal diurnal variation of PRL release. The effects of PRL and GH on the growth of the dispersed cells from the breast tumor was evaluated in monolayer culture using a recently developed microculture technique. Pharmacological quantities of GH produced significant increases in the number of population doublings of the breast tumor cells. Also, PRL concentrations present in the patient's circulation were demonstrated to significantly increase the number of population doublings of the breast tumor cells obtained in primary cultures. Thus, physiological concentrations of PRL stimulated the growth of breast tumor cells from this premenopausal patient.