Immunoglobulin-G from the sera of b9 rabbits was subdivided into three fractions (I, II and III) by anion exchange chromatography so that the most basic fraction (I) contained essentially λ-type and the most acidic (III) κ-type light chains. Upon oxidative sulfitolysis and gel filtration of IgG I, λ-type light chains emerged with a Kd value of 0.35–0.38 (LI) whereas IgG III yielded κ-type chains which eluted with a Kd value of 0.48 (LII). The λ-type chains were characterized on the basis of immunologic reactivity, presence of C-terminal serine, absence of a free N-terminal amino acid and the presence of 5 half-cystine residues. The κ-type chains were characterized immunologically and by the presence of 7 half-cystine residues. The major b9 IgG fraction (II) yielded an LI peak considerably larger than LII (75:25) with about equal quantities of λ- and κ-type chains. This LI fraction contained an average of 6 half-cystine residues.