A new human cell line was established from a prolactin (PRL) secreting pituitary adenoma. This cell line, designated as HPA, initially produced and secreted PRL, but the ability was decreased with increasing passage number. After about 30 passages in vitro, these cells had a short doubling time (14 h) and a low plating efficiency (9%). When a minimum of 10(5) cells was injected per mouse, virtually all athymic nude mice developed a slow growing, nonmetastasizing tumor at the injection site about 30 days after injection. PRL production by the HPA cells after Day 150 was demonstrated by immunocytochemistry as well as radioimmunoassay. In addition, cimetidine (10(-4) M) had a significant stimulatory effect on PRL secretion by 4-day-cultured HPA cells. When the HPA cells were incubated in the presence of 5.0 and 10.0 nM bromocriptine, the proliferation rate was inhibited to 53.4 and 25.1% of untreated controls, respectively. On the other hand, the same concentrations of bromocriptine did not affect the proliferation rate of YK cells derived from human immature teratoma of the ovary. In addition, bromocriptine inhibited significantly the growth rate of xenotransplanted HPA but not YK cells. These results suggest that bromocriptine inhibits specifically the proliferation of HPA cells.