Abstract Light chains devoid of b5 allotypic specificity have been isolated from rabbits suppressed by administration of anti-b5 antibodies. Chemical characterization of these b − chains has indicated that the carboxyl and amino-terminal regions have been considerable homology with myeloma lambda chains of man and mouse. Five half-cystine peptides labelled with ethyleneimine-1,2-[ 14C] have been isolated by gel filtration of Sephadex G-25 and G-50 and purified by chromatography on DEAE Sephadex A25, Dowex 50 X2, and paper electrophoresis at pH 6·5 or pH 3·6. Four peptides were obtained in reasonable yield and comparison of their amino acid sequence showed similarities with known structure of lambda chains in other species. A fifth peptide was isolated from a mixture of apparent variants and a partial sequence was determined. This peptide has been assigned to position of cysteine 90. The genetic markers c7 and c21 of rabbit lambda chains are probably not due to amino acid substitutions in the cysteine peptides of the constant region, since we found no variants.