Previous studies have failed consistently to detect either binding sites for exogenous IgG or endogenous IgG on human trophoblast by using conventional immunohistologic methods. The present study demonstrates specific binding sites for exogenous IgG on all trophoblast-derived cells. The binding sites were detectable with monomeric IgG but not IgG aggregates. The binding sites were detected more easily if cells were washed at an acid pH (5.0). The level of binding activity was greater in younger placental tissue, although expression of binding sites on cells in extraplacental membranes was age-independent. Complexing of bound exogenous IgG resulted in its dissociation from tissue binding sites. Endogenous trophoblast-associated IgG was readily detectable when anti-IgG reagents that produced minimal complexing were used, e.g., Fab fragments of rabbit antihuman IgG. High levels of both Fc gamma binding sites and endogenous IgG were detected in placentae as late as 30 wk gestation, but both were present only in low levels at term. The results demonstrate that both binding sites for exogenous and endogenous IgG are associated with the human trophoblast and support the theory that IgG transport through the human trophoblast is dependent on Fc gamma receptors.