We have investigated the effect of bromocriptin, a prolactin-lowering drug, on prolactin, estradiol and progesterone receptors in breast cancers. Doses of 2.5 mg per os of bromocriptin were given twice daily for 4 days before surgery. The efficiency of the treatment was evaluated by assaying the plasma prolactin level. The results obtained for the treated population (n = 30) was compared to those obtained for an untreated one (n = 120) during the same period. Bromocriptin does not increase the available prolactin receptors in breast cancer specimens. Unmasking in vitro receptors with MgCl2 3 M leads in both cases to an increase in prolactin receptors. The number of positive estradiol receptor tumors was increased after bromocriptin treatment. Neither the rate of positivity nor the level of progesterone receptors is changed by the treatment. This study demonstrates clearly the inefficiency of bromocriptin in unmasking in vivo prolactin receptors in breast cancer.