Chromosomal protein A24 has a unique structure inasmuch as it contains histone 2A and a nonhistone polypeptide the sequence of which has been partially determined. Comparative analysis of the ninhydrin-insensitive amino-terminal tryptic peptides of protein A24 and histone 2A and a quantitative analysis of their carboxyl-terminal amino acid indicated that protein A24 has two amino termini and one carboxyl terminus. The amino acid sequence analysis of tryptic peptide 17' of protein A24: (see text) showed it contains tryptic peptide 17 of histone 2A, Lys-Thr-Glu-Ser-His-His-Lys. Lysine 119, the amino terminus of this peptide, which is derived from the histone 2A portion of protein A24, is linked by an isopeptide bond to the carboxyl group of a glycine residue. Accordingly, the branched structure of protein A24 proposed is: (see text).