An in vitro assay method for predicting the hormonal response of primary cultured cells was used in 38 women with breast carcinoma. The response to 17 beta-estradiol (E2) was compared with other hormone treatments, such as tamoxifen (TAM), estracyt (EC) and bestrabucil (BB). The response to the E2-conjugated drugs (EC and BB) and TAM showed a good correlation with the response to E2 (p < 0.01). Coincidental rates of EC and BB response (stimulatory, insensitive, and inhibitory) with E2 response were 47% (16/34) and 53% (16/30) of cases, respectively. Cells showing an inhibitory response to E2 were also inhibited by EC and BB in 60% (6/10) and 100% (8/8) of cases, respectively. In premenopausal women, 28% (5/18) and 43% (6/14) of cases were inhibited by EC and BB, respectively, whereas in postmenopausal women 44% (7/16) and 56% (9/16) of cases showed inhibition with EC and BB, respectively. The inhibitory response to the E2-conjugated drugs was not significantly different between pre- and post-menopausal women. Cells resistant to inhibition by TAM were inhibited by EC and BB, respectively, in 17% (4/24) and 30% (6/20) of cases. These results indicate that E2-conjugated drugs may inhibit the growth of breast cancer cells and that their inhibitory actions might be different from those of TAM.