Human red cells from donor Pj carry the Sta blood group antigen and an unusual sialoglycoprotein of 24 kDa molecular mass tentatively identified as a hybrid molecule of the anti-Lepore type [Blanchard et al. (1982) Biochem. J. 203, 419-426]. This component is resistant towards proteinase treatment and was purified from trypsin-treated and chymotrypsin-treated Pj erythrocytes. The molecule is composed of 99 amino acid residues whose alignment was established following manual and automatic sequencing of cyanogen bromide, trypsin, chymotrypsin and V8 proteinase peptides. The polypeptide chain comprises residues 1-26/28 of glycophorin B and residues 59/61-131 of glycophorin A. The sugar composition resembles that of glycophorin B, indicating the absence of an N-glycosidic chain. Identical sequences were obtained from analyses of the 24-kDa component purified from unrelated St(a+) donors. These results support the hypothesis that glycoprotein Pj represents a B-A hybrid molecule which is encoded by a new gene product resulting from an unequal crossing-over between the genes coding for the polypeptide chains of the glycophorins A and B. The novel molecule carries both N and Sta blood group antigens. The N activity is clearly understandable from the sequence of the five N-terminal residues (Leu and Glu at positions 1 and 5 respectively). Inhibition studies with the untreated and chemically modified hybrid glycoprotein indicate that the Sta determinant is located within residues approximately 25-30 of the molecule, which corresponds to the newly formed sequence found neither in glycophorin A nor in glycophorin B.