The amount of 125I-lgG which bound to membranes isolated from the human placenta was competitively inhibited by the presence of increasing amounts of unlabeled IgG but not by unlabeled albumin. The relationship between membrane-bound and free IgG indicated the presence of membrane receptors with an appreciable affinity for IgG. Incubation of membranes with collagenase or neuraminidase did not result in appreciable reduction of IgG-membrane binding, indicating that neither intact collagen nor sialic acid play an important role in the binding. Placental surface membranes isolated by salt extraction bound 3.79 ± 1.78 (SD) pmol IgG/μg membrane protein, whereas membranes isolated by differential centrifugation bound only 1.61 ± 0.24 pmol/μg (p 4, 4.5 × 104 and 6.0 × 104 daltons. These findings are consistent with the existence of a limited number of receptors for IgG on placental membranes, including IgG receptors on the microvillus membrane of the syncytial trophoblast. The latter, in accordance with Brambell’s hypothesis, could be of importance in the transplacental transport of maternal IgG.