AbstractThe effect of cells from the liver of C3H fetuses syngeneic to splenic responder cells on an allogeneic mixed lymphocyte response (MLR) was studied. After fractionation on ficoll-hypaque, interface cells from corn oil (vehicle for BP) or normal fetal liver (FL) controls (CO), obtained from 17–19 days gestation, enhanced proliferation in the mixed lymphocyte culture (>2-fold), while cells from FLs transplacentally exposed to benzo(a)pyrene (BP) showed a decreased capacity for augmentation (>2-fold less than CO). Unfractionated CO-FL cells at 0.5×106did not augment proliferation, but at 0.10×256enhancement with control FL cells was significantly higher than with BP-FL cells. Pelleted cells from BP- and CO-FLs were severely suppressive at the higher dose, while at 0.25×106proliferation was augmented with CO-FL cells, but not affected with BP-FL cells. At doses of 0.1×106or less, no effect was observed for either control or BP-FL cells. These data indicate that: a) FL cells syngeneic to responder cells of an allogenic mixed lymphocyte culture (MLC) have an augmenting but not suppressive capacity on cell proliferation; b) in utero insult with BP modifies the capacity of FL cells to augment proliferation in the MLC.