Electrophoretic analysis of acid-soluble chromosomal protein isolated from the erythrocytes of the bullfrog Rana catesbeiana reveals that the nucleated erythrocytes contain five major histones (H1A, H2A, H2B, H3, and H4) and three minor histone-like proteins (H1B, R1, and R2). Histone 5, found as an additional major histone of avian erythrocytes, is not detected in the frog erythrocytes. Three minor components of the bullfrog erythrocytes, which are not present in the avian erythrocytes, have been purified to electrophoretic homogeneity and characterized by amino acid analysis, NH2-terminal analysis, tryptic peptide mapping, and immunological techniques. H1B extracted with 5% HClO4 along with H1A has a very similar amino acid composition and tryptic peptide map to H1o, a subfraction of lysine-rich histones found in nondividing mammalian cells. Microcomplement fixation also shows that H1B and bovine liver H1o share some common antigenic determinants. R1, a basic protein having a ratio of basic/acidic amino acids of 2.0 and 20 mol % lysine, is distinguished from any chromosomal proteins characterized so far on the basis of electrophoretic mobility and amino acid composition. On the other hand, R2 is identified as protein A24 on the basis of its electrophoretic mobility, amino acid composition, and tryptic peptide map. Since H1o and protein A24 are considered to be involved in the inhibition of DNA replication and RNA synthesis, respectively, H1o-like protein and protein A24 in the frog erythrocyte lacking H5 may have central roles in genetic inactivation during erythrocyte maturation.