The amino acid sequence of the amino-terminal 111 residues (variable region) for the light chain of the homogeneous rabbit antipneumococcal type III polysaccharide antibody 3368 was determined. This sequence was obtained principally through automated Edmann degradations of the intact light chain and of peptides generated by tryptic digestion of the citraconylated light chain. With these methods only 2 mumol of purified light chain was required to determine the reported sequence. When compared with the light chains of four other antipneumococcal type III polysaccharide antibodies, the 3368 light chain exhibits a unique sequence in those segments of the variable region that contribute to formation of the antigen binding site (complementarity-determining regions) (10 or 11 residue differences in 12 positions). The 3368 light chain also demonstrates an insertion of three residues relative to the other four light chains in the complementarity-determining region at positions 89 to 98. These five light chains have greater than 80% sequence homology for the portion of the variable region which is not involved in antigen binding (framework).