The placenta is a major source of tumor necrosis factor-alpha (TNF alpha) and is rich in TNF alpha receptors. TNF alpha inhibited hCG secretion by normal chorionic villi at 6 weeks gestation, but chorionic villi contain the heterogenous cell population. To investigate the direct effects of TNF alpha on trophoblasts, the NUC1 choriocarcinoma cell line was used as an in vitro placental cell model. TNF alpha also inhibited hCG secretion by NUC1 cells at concentrations from 1-100 U/mL. TNF alpha at concentrations of 1, 10, and 100 U/mL significantly decreased hCG secretion for 24 h to 88%, 81%, and 71% of the control level, respectively. The expression of hCG beta mRNA was also decreased to 23% of the control level after 24 h of TNF alpha treatment (100 U/mL). Inhibition occurred after 12 h of TNF alpha treatment (100 U/mL). Two measurements, trypan blue dye exclusion and 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl tetrazolium bromide reduction, revealed that these inhibitory effects were not due to the cytotoxic activity of TNF alpha on NUC1. In conclusion, TNF alpha reduces hCG secretion in vitro.