Fetal passive immunity is acquired by transfer of maternal IgG through the placental syncytiotrophoblast and endothelium; few and contradictory data exist for IgG transcytosis in human placental endothelial cells (HPEC). In this study, we tested the binding and internalization of IgG by cultured HPEC and the expression of FcgammaRs. Biochemical analysis and microscopy revealed that the binding of IgG occurred through the Fc portion of the molecule and was greater on the basolateral than on the apical cell surface. IgG binding and internalization were saturable and the data calculated from Scatchard plot for IgG surface binding indicated a single interaction with an apparent K(d)of 2x10(-7)M. During 3 h of chase, approximately 10 per cent of IgG was released in an intact form in the medium. By electron microscopy, IgG was detected on HPEC surface, internalized in endothelial caveolae and within endosomal compartments. RT-PCR, blotting and microscopy failed to detect the presence of FcgammaRI-III in HPEC. However, the specific radioiodination and affinity chromatography revealed the presence of a 55 kDa-IgG binding polypeptide on cell surface. These findings indicate that HPEC (i) take up and internalize IgG via a receptor mediated process; (ii) bind IgG prevalently on the basolateral surface via the Fc fragment of the molecule and (iii) exhibit a novel FcgammaR of 55 kDa on the cell membrane.