The majority of human ovarian cancers originate from the surface epithelial (OSE) cells that surround the ovary. The incidence of OSE cancer is correlated with the number of ovulations that occur during fertile life. OSE cells remain quiescent but undergo rapid mitotic activity after ovulation to repair the wound. This increase in mitotic activity following each ovulation may give rise to mutations that make the OSE susceptible to malignant transformation. Steroidogenesis-inducing protein (SIP), a protein isolated from human follicular fluid obtained from hyperstimulated ovaries, is a potent mitogen for several gonadal cells. To investigate the possibility that SIP may be involved in the proliferation of OSE cells, we have studied its effects on DNA synthesis in seven cell lines derived from OSE carcinomas (HEY, MLS, SKA, OW-1, SAU, NIH:OVCAR-3, and Caov-3). The cells were cultured in serum-free medium in the presence of SIP for 18 hr, followed by incubation with [3H]thymidine for 4 hr. The radioactivity incorporated into the DNA was measured. SIP stimulated DNA synthesis in six of the cell lines. HEY, SKA, MLS, and OVCAR3 were most responsive to SIP. Interactions between SIP and other growth factors and cytokines known to be present in follicular fluid (EGF/TGFalpha, TGFbeta, FGF, IGF-1, IL-1beta, and TNFalpha) were also investigated in HEY and SKA cells. EGF/TGFalpha and IGF-1 potentiated the effects of SIP. TGFbeta had no effect on SIP, and/or EGF/TGFalpha stimulated DNA synthesis. Other growth factors which were tested in this study had no effect on DNA synthesis in SKA cells. Dibutyryl cyclic-AMP blocked the effects of SIP on DNA synthesis. We conclude that SIP is a potent mitogen for OSE cell lines and together with TGFalpha and IGF-1 may be involved in the proliferation of normal OSE cells after ovulation. Since SIP is obtained from the preovulatory follicle, it may represent a link between the number of ovulations and the increased incidence of OSE cancers.