Maternal transport of immunoglobulin to the newborn mammal is important for immune defense during the first weeks of independent life. Receptors for the Fc portion of IgG mediate the transfer of immunoglobulin from milk to the bloodstream of newborn mice and rats, by passage through intestinal epithelial cells. Neonatal Fc receptors (FcRn) isolated from intestinal epithelial cells of suckling rats bear a striking resemblance to class I histocompatibility molecules. The heavy chain of FcRn has sequence similarity in three extracellular domains to the corresponding domains of class I molecules, and the light chain of both types of molecules is beta 2-microglobulin. To facilitate biochemical characterization and crystallization of FcRn, we have expressed a secreted form, as well as two different lipid-linked forms solubilizable by phospholipase treatment. The lipid-linked forms are heterodimers consisting of beta 2-microglobulin and the extracellular portion of the heavy chain and are anchored to the membrane by a phosphatidylinositol linkage attached to either the heavy chain or beta 2-microglobulin. Cells expressing either lipid-linked form bind rat Fc, reproducing the known physiological pH dependence of binding. Secreted FcRn has been purified in yields up to 40 mg/liter from cell supernatants. Circular dichroism spectra of soluble FcRn appear similar to spectra of class I MHC molecules, suggesting that the similarities in primary sequence extend also to a similarity in secondary structure. Soluble FcRn crystallizes in a form amenable to a structure determination by x-ray diffraction methods, which will ultimately allow a detailed comparison of the two types of molecules.