Human, rat, and mouse placentas and rat and mouse intestines were homogenized in buffered saline, and fraction consisting primarily of cell membranes was separated from each of the homogenates by differential centrifugation. Human, bovine, and guinea pig IgG, and human IgE, Bence-Jones protein, serum albumin, insulin, and growth hormone were labeled with (131)I or (125)I, and the binding of these proteins by the cell membrane fractions was investigated. Rat and mouse sucklings were given labeled proteins intragastrically, and the amount of each protein absorbed after a given interval of time was determined. It was found that the degree and specificity of protein binding by the cell membrane fractions from human and murine placentas strikingly paralleled the relative rate and specificity of protein transport from mother to fetus in the respective species at or near term. Similarly, the degree and specificity of protein binding by the cell membrane fractions from suckling rat and mouse intestines tended to parallel the rate and specificity of protein absorption from the gastrointestinal tract in these animals. However, some discordance between protein binding and protein transport was also observed. The data suggest that: (a) the binding of a protein by specific receptors on cell membranes may be a necessary first step in the transcellular transport of the protein; (b) specific protein binding by cell receptors does not ensure the transport of that protein across the tissue barrier; and (c) specific transport mechanisms other than or in addition to specific cell membrane receptors are involved in the active transport of proteins across the human or murine placenta or the suckling murine intestine.