The variable portion of a human λ-type Bence Jones protein (Lee) has been prepared by limited tryptic digestion. Its amino acid sequence is determined from analysis of tryptic peptides, CNBr cleavage fragments prepared from the reduced-aminocthylated protein. Peptides are aligned by homology to other λ-type proteins. The Lee L-chain has an unusual extension of two amino acid residues (Val-Thr) at the N-terminus. An idiotypic antibody of Lee can react with two other λ-type Bence Jones proteins, Meg (V_λⅣ) and Cap (V_2 Ⅲ). Application of the principle of Chou and Fasman to the sequence data indicates that two portions on the surface of the Ⅴ-region have identical secondary structures similar to the portions of the Meg and Cap λ-chains. This appears to relate to the idiotypic similarities of the three proteins. Its amino acid sequence is determined from analysis of tryptic peptides, CNBr cleavage fragments prepared from the reduced-aminocthylated protein. Peptides are aligned by homology to other λ-type proteins. The Lee L-chain has an unusual extension of two amino acid residues (Val-Thr) at the N-terminus. An idiotypic antibody of Lee can react with two other λ-type Bence Jones proteins, Meg (V_λIV) and Cap (V_2 Ⅲ). Application of the principle of Chou and Fasman to the sequence data indicates that two portions on the surface of the Ⅴ-region have identical secondary structures similar to the portions of the Meg and Cap λ- chains. This appears to relate to the idiotypic similarities of the three proteins.