Explant and cell culture methodologies are frequently employed in the investigation of the mechanisms that mediate placental hormone production. Recent reports suggest the presence of unknown regulatory factors in maternal serum that may impact significantly on the regulation of these biosynthetic pathways. The present study, therefore, determined the effects of sera obtained from pregnant women in the second to third trimester (PWS), nonpregnant women (NPWS), and men (MS) as well as commercially prepared FBS on hCG, estradiol, and progesterone release into medium by cultured human trophoblast cells. Placental villous tissue was enzymatically dispersed, and cytotrophoblast cells were purified via density gradient centrifugation and cultured (37 C; 90% air-10% CO2) in DMEM with 10% PWS, NPWS, MS, or FBS. All cytotrophoblast cultures aggregated and progressed to syncytial forms, although cells cultured with PWS exhibited notably larger multinucleated syncytial elements by 48 h in culture than cells cultured with FBS. Significant increases (P < 0.05) occurred in hCG, estradiol, and progesterone release due to the progression of cytotrophoblasts into the syncytiotrophoblast phase in all cultures. The quantity of hCG release was unaffected by serum origin. Cells cultured with human serum released greater (P < 0.05) amounts of estradiol than cells cultured with FBS. Cells cultured with MS released more (P < 0.01) estradiol than cells cultured with either PWS or NPWS, in a ratio to the concentration of endogenous androgen precursor available. Progesterone release was greater (P < 0.01) for PWS-cultured cells than for FBS-cultured cells. Progesterone release by NPWS- and MS-cultured cells was intermediate. Syncytiotrophoblasts cultured with PWS released approximately 3-fold more (P < 0.01) progesterone than syncytiotrophoblasts cultured with FBS and low density lipoprotein cholesterol, although the concentrations of available cholesterol substrate were similar. Culture of cells in steroid-depleted or lipoprotein-depleted PWS or FBS resulted in similar decreases (P < 0.01) in estradiol and progesterone release, respectively. In summary, increased estradiol release by placental cells cultured in intact human serum was attributed to aromatizable androgens, whereas enhanced progesterone release by cells cultured in human serum could be only partially attributed to higher concentrations of low density lipoprotein cholesterol substrate in human serum. Evidence of increased syncytial maturity and progesterone release by PWS-cultured cells may indicate the presence of undefined serum-borne regulators, which is enhanced during pregnancy.