Electrophoresis patterns of cyanogen bromide (CNBr) peptides from the proteins of the cowpea (SBc), bean (SBb), Mexican (SBm), and Ghana (SBg) strains of southern bean mosaic virus showed a marked similarity between SBc and SBg and between SBb and SBm. A highly basic CNBr peptide from SBc, cCB-1, was isolated by Sephadex and ion-exchange chromatography. The size and amino acid composition of cCB-1 was similar but differed from that of the N-terminal CNBr peptide bCB-1 from SBb described previously.Antisera to cCB-1 or bCB-1 conjugated to tomato bushy stunt virus (TBSV) were used in gel diffusion tests with virus particles of the four SB strains. With the bCB-1–TBSV conjugate antiserum, SBb gave a reaction of identity with SBm and reactions of partial identity with SBc and SBg. With the cCB-1–TBSV conjugate antiserum, SBc gave a reaction of identity with SBg and reactions of partial identity with SBb and SBm. Cross absorption of bCB-1–TBSV conjugate antiserum with TBSV yielded an antiserum that reacted with SBb but not with TBSV. Cross absorption with bCB-1 yielded an antiserum that reacted with TBSV but not with SBb.Limited tryptic proteolysis of SBc in 0.01 Msodium phosphate buffer containing 0.01 Methylenediaminetetraacetic acid, pH 8.0, resulted in the formation of viruslike particles and smaller spherical particles. Sixteen peptides were isolated from these digests and the compositions of most of them were similar to but not identical to peptides isolated from limited tryptic proteolysis of SBb.