Most oral contraceptives (OC) contain a progestin in combination with an estrogen, and the progestin component in OC includes one of the following 19-nortestosterone derivatives: norethynodrel; norethindrone; or norgestrel (levonorgestrel). It is well known that estrogens promote the growth of breast cancer. However, progestins have recently also been implicated in the development of breast cancer. We have compared and contrasted the ability of synthetic progestins to stimulate the proliferation of cultured human breast cancer cells and examined their possible mechanism of action. We found that some progestins used in OC were able to stimulate the growth of estrogen receptor-positive (ER+) MCF-7 and T47DA18 human breast cancer cells but not ER- MDA-MB-231, BT-20, and T47DC4 human breast cancer cells. However, two other progestins, MPA and R5020, which are not used in OC, were either not able to stimulate or only slightly stimulated growth. The potency of norethynodrel [median effective dose (EC50) = 4 x 10(-8) M] and norethindrone (EC50 = 3 x 10(-8) M) was greater than norgestrel (EC50 = 2 x 10(-7) M) in MCF-7 cells. E2 (EC50 = 8 x 10(-13) M) was an even more potent stimulator of growth. More importantly, the progestin-induced growth stimulation was blocked by the antiestrogens 4-hydroxytamoxifen and ICI 164,384 but not the antiprogestin 17 beta-hydroxy-11 beta-(4-dimethylaminophenyl)-17 alpha-(1-propynyl)-estra-4, 9-dien-3-one (RU486). To determine whether the proliferative action of progestins was mediated through the ER, cells were transfected with a chloramphenicol acetyltransferase reporter gene containing an estrogen response element derived from vitellogenin 2A gene. The progestins which stimulated the growth of breast cancer cells also increased chloramphenicol acetyltransferase activity. The induction of chloramphenicol acetyltransferase activity was blocked by the addition of the antiestrogens 4-hydroxytamoxifen and ICI 164,384 but not the antiprogestin RU486. This study provides direct evidence that the 19-nortestosterone derivatives in OC have estrogenic properties and suggests that activation of ER, but not progesterone receptor, is the growth-stimulatory mechanism for these synthetic progestins. Our results may help to explain the conflicting evidence linking OC and breast cancer risk. A rigorous evaluation of the "total" estrogenic potential of OC might produce a better correlation with breast cancer risk.