In maternofetal transfer of passive immunity, IgG is transferred across cells of fetal membranous tissues. Inasmuch as the first step in the process, the binding of IgG to receptors on the cell membrane, appears to be the factor which determines the overall specificity, an investigation will be made into the nature of the cell membrane receptors. It will be in two parts: the binding characteristics of IgG receptor substance, and a study of its isolation and biochemical properties. Measurements of binding characteristics of receptor substance on rabbit yolk sac membrane or homogenates of the apical brush border cells or vesicles from them will be carried out. The influence of hydrolases on binding properties will be studied. Solubilization of receptor substance from rabbit yolk sac membrane will be done with brief exposure of yolk sac membrane or apical brush border membrane vesicles to the action of papain. Release of IgG receptor substance from the glycocalyx will be monitored by I125-IgG binding. Gel filtration, molecular weight, association with enzyme components of the glycocalyx and the question of the importance of sialic acid for IgG binding will be among the properties studied. Measuremenns for IgG will be made of the binding constant, KA of the soluble receptor substance. BIBLIOGRAPHIC REFERENCES: Schlamowitz, M., Kaplan, M., Shaw, A. R., and Tsay, D> D.: J. Immunol., 114: 1590 (1975). Preparation and Characterization of Rabbit IgG Fractions. Schlamowitz, M., Hillman, K., Lichtiger, B., and Ahearn, M. J.: J. Immunol., 115: 296 (1975). Preparation of IgG-Binding Membrane Vesicles from the Microvillar Brush Border of Fetal Rabbit Yolk Sac.