Eight monoclonal antibodies from mouse hybridomas raised to normal human erythrocytes were tested with a panel of null-type erythrocytes, enzyme-treated normal cells, and by inhibition with human erythrocyte sialoglycoproteins. Two antibodies reacted poorly or not at all with RhNULL cells. These antibodies are of considerable interest since it may be possible to use them to elucidate the chemical nature of the antigens of the Rhesus blood group system. Four other antibodies were inhibited by sialoglycoprotein preparations. The antigens recognized were, respectively, two different determinants on the major sialoglycoprotein alpha (glycophorin A) and one determinant which is probably common to sialoglycoproteins alpha and delta (glycophorins A and B). Another antibody had anti-Wrb specificity. One of these antibodies is of considerable potential value for the further characterization of erythrocyte sialoglycoproteins.