The human myeloma protein Boh (gamma 2, lambda) was isolated and completely reduced and aminoethylated. The light chain was obtained by chromatography on Sephadex G-100 in 4 M guanidine HC1. The amino-terminal sequence on the blocked light chain could be determined by automatic sequence degradation after PCAase treatment. Twenty-one peptides were isolated from a tryptic digest and 12 peptides from a chymotryptic digest. The sequence determination on these peptides was performed by automatic sequencing methods. The light chain of Boh protein belongs to the lambda II subgroup. Unique substitutions have been found at position 8 (Arg) and position 62 (Tyr). Furthermore, the Boh light chain has six cysteine residues, the additional (sixth) cysteine being adjacent to the invariable intrachain-S-S linking cysteine at position 91. Sequence comparison of lambda II proteins reveals a high degree of homology emphasizing the biologic significance of the hypervariable region sequences;