The transfer of passive immunity in the rabbit is mediated by the fetal yolk sac membrane (YSM) and is initiated by the specific binding of IgG to receptors on the microvillar surface of the endoderm from the YSM. This report describes the preparation of suspensions of endodermal absorptive cells of the YSM and their use in equilibrium binding experiments to characterize the nature of the binding reaction. Equilibrium binding is achieved in 4 h at 4 degrees C. The system is more rapid than, affords greater reproducibility of binding data than, and utilizes only about 1/10 the amount of YSM and ligand as the YSM disc assay system (Tsay and Schlamowitz, 1975) used previously. A Scatchard plot of the binding data over a wide range of IgG concentration was non-linear implying the presence of at least 2 binding elements. Apparent binding constant values for the stronger and weaker binding components in this population differed by about 50-fold. For the weaker binding system, binding decreased when temperature was increased indicating that the reaction was not entropy-driven (i.e., dominated by hydrophobic 'forces') and that ionic interactions might be a major factor. At low ionic strengths the measurement of specific binding was complicated by the effects of secondary ionic interactions. At physiological ionic strength the binding of IgG was species-specific.