The only fetal cell membrane exposed to the mother in the mouse yolk sac placenta is the apical membrane of the endodermal epithelial cells. In yolk sac preparations in vitro, this apical membrane was exposed to reagents or cells in the incubation medium. By using several techniques we were not able to detect fetal major histocompatibility complex (MHC) antigens in this membrane. Immunoferritin labeling with and without prefixation and after neurominidase and trypsin digestion indicated that the apical membrane could contain no more than approximately 1% of the H-2 complex antigens that were present on peritoneal macrophages. Incubation of yolk sac preparations in anti-H-2 complex antiserum and complement had no cytotoxic effect on the endodermal epithelium, nor did incubation in an excess of alloreactive lymphocytes. Dissociated preparations of prefixed yolk sac contained endodermal epithelial cells and vascular endothelial cells whose entire surface membranes were exposed to the medium. H-2-complex antigens were not detected by immunoferritin labeling in either the apical or the laterobasal membrane of the yolk sac endoderm, but they were present in low density on the vascular endothelium. Also, incubation of unfixed, dissociated cells in anti-H-2-complex serum and complement had no detectable cytotoxic effect on endodermal epithelial cells. These observations indicate that H-2 antigens are sparse or absent in both the apical and laterobasal membranes of endodermal epithelial cells. The deficiency of MHC antigens in the apical membrane may account for the failure of sensitized females to reject the yolk sac, whereas the composition of the laterobasal membrane is probably less important to maternal-fetal relations. The present observations are consistent with labeling studies of adult-lining epithelial cells, which indicate that self-marker MHC molecules are absent from the apical membranes oriented toward the outside world and variably expressed in the laterobasal self-side membranes. It is suggested that the corresponding exclusion of fetal self-marker molecules from the apical membranes of some kinds of placental epithelia would deprive the mother of target sites for an alloimmune reaction at the maternal-fetal interface.