The trophoblastic tissue and endothelium of fetal stem vessels in cryostat sections of first-trimester and term human placentae adsorbed erythrocytes sensitized with IgG antibodies of human, rabbit, and guinea pig origin. Native and heat-aggregated human and rabbit IgG, and human IgG subclasses except IgG2 inhibited the hemadsorption. However, the Fc fragment of IgG2 inhibited the hemadsorption similarly to the Fc of pooled IgG. No inhibition was obtained with other immunoglobulin classes or fragments. The inhibitory activity of IgG and Fc fragments was abolished by partial reduction and alkylation, indicating that inter-heavy-chain disulfide bonds are required for the binding. Immune complexes prepared at slight antigen excess showed an eightfold stronger inhibition than antiserum alone. Both receptors were sensitive to formaldehyde, periodic acid, and heat. The Fcgamma receptors on the trophoblasts and on the endothelium in human placentae are apparently similar.