Evidence indicating abnormal biological behavior of trophoblasts has been seen in preeclamptic patients, but the mechanism is still unknown. We have previously shown that endothelial injury and neutrophil activation are-induced by certain factors in preeclamptic sera. We investigated the effect of sera from eight preeclamptic and 11 normal pregnant women on cellular proliferation and viability of trophoblasts. using H-3-thymidine incorporation and the trypan-blue dye exclusion test, respectively. Five of eight preeclamptic sera, but none of the normal pregnant sera, inhibited H-3-thymidine incorporation. The trypan-blue test revealed the sera reduced cellular viability. Gel permeation showed that the greatest growth-inhibitory activity corresponded to a molecular weight of 50 kDa. The serum-mixing test revealed this permeation and inhibitory preeclamptic sera suppressed the growth-promoting activity of normal pregnant sera in a dose-dependent manner. These results suggested the presence of certain factors in some preeclamptic sera that can affect cellular behavior of human trophoblasts.