The authors studied the effects of soluble syncytiotrophoblast extract (STE) on the proliferative responses of lymphocytes to different lectins (PHA and Con A) and to allogeneic cells in one-way mixed lymphocyte cultures. STE suppressed lymphocyte reactivity to lectins and to allogeneic cell cultures. The inhibitory effect was dose dependent. Increased concentrations of lectins failed to overcome the inhibitory effect of STE. Lymphocytes preincubated with STE for 18 hr, then washed and exposed to lectins still exhibited an inhibition of cellular proliferation. STE added to lymphocyte cultures at various times in the presence of both mitogens or of allogeneic cells continued to inhibit lymphoproliferative responses even when it was added after 43 hr of cell culture. Furthermore, STE was able to reduce the spontaneous proliferation of tumor cell lines K562 and LHN13 maintained in vitro culture prior to testing. In all cases, the inhibition observed was not due to lymphocytotoxicity or to tumor cell mortality. The inhibitory effect of STE on the proliferative responses of lymphocytes to stimulants may be due to a cytostatic effect that may represent a contributing factor in the nonrejection of the fetus by a competent immune system.