We have studied the estradiol sensitivity of primary human breast carcinomas in organ culture in a prospective pilot series of 109 tumors. The effect on plasminogen activator (PA) production was used as the end-point of estrogen action. We found that: (i) All tumors secreted detectable levels of urokinase-type PA (uPA); the level of basal uPA production was markedly heterogeneous but showed a weak association with the level of estrogen receptor positivity (p = 0.049). (ii) Only 23.5% of the tumors secreted tissue-type PA (tPA) in addition to uPA; a higher proportion of these tumors had histological characteristics indicative of good prognosis (18% vs. 3% of tumors secreting only uPA). (iii) Estradiol modulated uPA production and this effect was receptor-mediated. (iv) Responsiveness to estradiol was limited to a subset (25 of 60 or 41.7%) of estrogen and progesterone-receptor-positive tumors. (v) Of 20 evaluable patients with lymph-node and receptor-positive breast cancer who received adjuvant anti-estrogen therapy, 11 were identified as estradiol-sensitive by the in vitro PA assay; of these, 10 had no evidence of disease after a median follow-up period of 3+ years. In contrast, of 9 patients with tumors identified as estradiol-insensitive, 4 developed metastases within 3+ years of follow-up. (vi) Consistent with the previously reported inhibitory effect of corticosteroids on uPA production in organ cultures of human tumors, the basal culture level of uPA produced by tumors from patients receiving corticosteroids at the time of surgery was significantly lower than the level of uPA in the remaining tumors (p = 0.029). Also, tumors from patients receiving thyroid hormone, known to stimulate uPA in vitro, showed a slight trend toward increased production of uPA. These results show that hormone effects on tumor PA production are qualitatively similar in organ culture and in the host. This and the emerging individual correlation between sensitivity to estradiol in vitro, as determined by PA, and the clinical effectiveness of anti-estrogen therapy, underscore the potential usefulness of the organ culture approach.