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St(a+) red cells contain a ficin-resistant hybrid sialoglycoprotein (SGP) consisting of the amino terminus of Ss sialoglycoprotein (Ss SGP) and the carboxyl terminus of MN sialoglycoprotein (MN SGP). Ficin-modified St(a+) but not St(a-) red cells were agglutinated by monoclonal antibodies NN5 (anti-N) and 31 (detecting a sialic acid-dependent determinant on the SGPs). SDS-polyacrylamide gel electrophoresis showed three periodic acid-Schiff-staining protein bands in ficin-modified St(a+) but not St(a-) red cell-membranes; these bands bound monoclonal antibodies NN5 and 31 on Western blots. Using Scatchard analysis of 125I-NN5 IgG binding to M+N- red cells, estimates were obtained of 25,000 Ss SGP molecules per red cell per s allele, 33,000 Ss SGP molecules per red cell per S allele, and 75,000 hybrid SGP molecules per red cell per hybrid allele. These results are consistent with the expression of the hybrid SGP gene at a level intermediate between the MN SGP gene and the Ss SGP gene. Monoclonal antibodies NN5 and 31 may be useful in screening ficin-treated red cells for hybrid SGP variants.