Cyanogen bromide cleavage of the heavy (alpha) chain of protein 315 (an immunoglobulin A mouse myeloma protein with anti-dinitrophenyl activity) yielded five fragments of which one (CN2), with 156 residues, contained the chain's entire variable region. Determination of the amino-acid sequence of CN2 showed that: (1) the variable region has appreciable homology (about 33% identities) with the variable region of the light chain from the same molecule; and (2) the constantregion sequence immediately following the probable transition from variable to constant domains is the same in the protein-315 a as in human -yl and p chains (-ValSer-Ser-). The sequence of the cyanogen bromide octapeptide (CN5) from the carboxy terminus of the protein-315 heavy chain closely resembles the corresponding segments ofhuman a and ju chains.Multiple lines of evidence show that the ligand-binding specificities of antibody molecules are determined by about 107 residues at the amino terminus of light chains and about 115 at the amino terminus of heavy chains (called VL and VH regions, respectively) (1-3). The complete amino-acid sequences for VL and VH of the same molecule have been established for two myeloma proteins (4-6), but their ligandbinding specificities are not known.The purpose of this paper is to report the amino-acid sequence of the variable region of the heavy chain (H315) from protein 315. This sequence and that previously reported for the light chain from the same molecule (7) (9). The fragments obtained from cyanogen bromide degradation of the heavy chain were subjected to sequence analysis in the Beckman sequenator and by the dansyl-Edman procedure (10, 11). Several of the fragments were degraded further with trypsin, chymotrypsin, and thermolysin into smaller peptides, which were purified by ion exchange chromatography on Dowex resins (12), gel filtration, paper chromatography, and high-voltage electrophoresis. The amino-acid sequences of these peptides were determined with the aid of dansyl-Edman degradation and by digestion with carboxypeptidases A and B. Analysis of heavy chains from intact molecules that had been labeled with a specific affinity-labeling reagent ([14C]bromoacetylDnp-L-lysine) aided in identification of the fragment with the variable region (13).Cyanogen bromide cleavage of the fully reduced, aminoethylated heavy chain yielded five fragments that were purified largely by gel filtration on Sephadex (G-50 and G-100) in 5 M guanidine-HCl (Fig. 1). This report focuses on the 2nd and 5th fragments, CN2 and CN5, which correspond, respectively, to a large amino-terminal (variable) segment and a small carboxy-terminal (constant) segment of the chain. The other fragments, all derived from within the constant region, will be described elsewhere: here we note only that CN1 has approximately 228 residues, CN3 is a mixture of peptides, including one with carbohydrate, and CN4 is an octapeptide.CN2 contains the entire VH region and the adjoining portion of the constant region, including the latter's first ...