Expression of Fc gamma receptors on human placental trophoblast was investigated by immunostaining and immunoblotting using a panel of Fc gamma receptor monoclonal antibodies (mAb). Fc gamma receptors typical of other cell types were not detected on syncytiotrophoblast in term placentae when transplacental IgG transport was maximal. Unexpectedly, however, and by contrast with term, all Fc gamma receptor III mAb tested bound to first trimester placental syncytiotrophoblast by immunostaining. Reactivity was relatively restricted and varied between specimens. Fc gamma receptor III products of 41,000-45,000 and 49,000-52,000 MW were consistently detected on first trimester trophoblast membranes by immunoblotting and levels of these products were greatly reduced following treatment with phosphatidylinositol-specific phospholipase C, suggesting that the early trophoblast Fc gamma receptor III is glycosyl-phosphatidylinositol (GPI) linked. The mAb Leu-11b behaved differently to other anti-Fc gamma receptor III mAb examined. By immunostaining, Leu-11b bound to syncytiotrophoblast at term and detected both syncytiotrophoblast and underlying cytotrophoblast in the first trimester. In addition to the GPI-anchored Fc gamma receptor III in first trimester, Leu-11b also detected a 74,000 MW component on both first trimester and term trophoblast membranes by immunoblotting. Thus trophoblast appears to express a GPI-anchored Fc gamma receptor III in first trimester but not term placentae. With the exception of the 74,000 MW Leu-11b-defined product whose function is unclear, currently available Fc gamma receptor mAb appear to be incapable of detecting the protein involved in IgG transport during the later stages of gestation.