Amyloid subunit proteins related to the λ IV subgroup of immunoglobulin light chains have not been previously reported. We have determined the amino acid sequence of an AL amyloid protein BAK and shown that it has the structure typical of λ IV light chain proteins. This protein, which was isolated from the spleen of a patient with AL amyloidosis, has 111 residues in the variable domain and also includes the first tryptic peptide of the constant domain for a total of 130 residues. Comparison of the primary structure of this protein with the only other completely characterized λ IV protein (SH) reveals that they are highly homologous with only one amino acid change in FR1, two changes in FR2, and one change in FR3. The CDR regions also show few changes, with only three in CDR1, one in CDR2, and five in CDR3. To test the hypothesis that the formation of AL amyloid is related to changes in the FR regions which could affect molecular aggregation, the structure of BAK was compared with the myeloma protein SH with respect to the presumed tertiary structure. Only limited amino acid substitution was found in the surface positions that might affect intradimer and interdimer aggregation. These included an isoleucine for leucine change at position 43 and phenylalanine for valine at 45, which may affect intradimer interaction and a change of histidine to asparagine at position 67.