Immunoelectron microscopy of IgG molecules in human mature placenta has shown that IgG bound to microvillar surfaces and the inner wall of endocytotic vesicles of syncytiotrophoblasts. The endocytotic vesicles, containing both bound and unbound IgG molecules, tended to fuse with each other or with other cellular organelles, particularly with lysosomes. The phagolysosomes were more abundant in the basal regions of the cells. Apparently some IgG molecules were not digested by lysosomal enzymes. Vesicles with residual IgG were found to fuse with the basal and basolateral cell membrane and to discharge their contents into the extracellular space by exocytosis. It is suggested that IgG molecules were transported through the trophoblastic basement membrane and the interstitial space by diffusion to the endothelial basement membrane. The IgG molecules then migrated into the fetal vascular lumen via endothelial gaps and interendothelial spaces.