To investigate the effects of placental serum to in vitro culture of the normal human lymphocytes the peripheral blood lymphocytes were isolated by ficoll-hypaque separation method in 20 healthy human adults. The blast transformation respone of lymphocytes to mitogens were observed by stimulation with PHA($25{mu}g/ml$) and Con A($25{mu}g/ml$) using RPMI 1640 media containing 20% placental serum(PS), tetal calf serum(FCS), or humans AB serum(AB). And one-way mixed lymphocyte culture test was performed between these unrelated person compounded into stimulators and responders to investigate the effect of placental serum. The following results were obtained. 1. In 20 experimental cases, these were no significant diffenence between FCS, AB, and PS in untreated control cultures. 2. In PHA-treated cultures, whereas the blast transformation rate of the FCS groups and AB groups were $40.8{pm}4.3%$ and $44.6{pm}4.3%$ respectively, that of PS groups were $21.7{pm}3.4%$, Similar results were obtained in Con A-meated cultures. Therefore, placental serum inhibited the mitogenic response of lymphocytes significantly. 3. In MLC tests, stimulation index of the FCS groups and AB groups were $18.5{pm}3.5$ and $20.1{pm}3.3$ respectively. But placental serum inhibited MLC response of lymphocytes significantly($7.4{pm}1.9$).