ABSTRACT: Cells from mice alloimmunized in the presence of placental extract were coinjected with cells from mice conventionally alloimmunized in the footpad of virgin female recipients. The contralateral footpad received a control twice the dose of cells from alloimmunized mice. Cells from the popliteal lymph nodes were harvested on day 3, and pulsed in vitro for measurement of proliferative capacity with 3H thymidine. The response of lymph nodes was compared (homo- vs contralateral). The cells from mice alloimmunized with placental extract in conjunction with alloantigens displayed a marked suppressive capacity of the local graft versus host (GVH) potential of cells from mice conventionally alloimmunized. It is suggested that this local GVH assay represents a quick, objective assay for suppressor-cell induction by placental products.