The minor red cell sialoglycoproteins--beta and gamma (also known as glycophorin C)--are believed to be important to the structural integrity of red cells. The absence of sialoglycoproteins alpha and delta, as seen in En(a-) and S-s-U- cells, respectively, results in cells with normal morphology, but the absence of sialoglycoproteins beta and gamma is associated with elliptocytosis. However, cells that lack Gerbich (Ge) antigens but have an abnormal sialoglycoprotein reactive with antibodies to beta have normal morphology. The authors used a monoclonal antibody specific for beta to explore the nature of this abnormal sialoglycoprotein and its interactions with other membrane proteins. The beta-like sialoglycoprotein of Ge-Yus- red cells appears to react poorly with some antibodies due to steric hindrance by other molecules of sites normally available to immune agglutinins. This steric hindrance is not due to a single interaction with either sialoglycoprotein alpha or delta or band 3. Furthermore, steric hindrance by other molecules does not account for the lack of Ge antigens on these cells.