The amino acid sequence of the constant region of the immunoglobulin lambda-chain of the rabbit (C lambda) has been determined. This chain was obtained from a homogeneous anti-(type II pneumococcal polysaccharide) antibody fraction raised in Basilea rabbit 4548. The C lambda-region sequence was established by the analysis of tryptic and some overlapping chymotryptic peptides and partly by homology with known C lambda-region sequences of lambda-chains from other species. Peptides were isolated by a combination of methods including high-voltage paper electrophoresis, gel filtration and high-pressure liquid chromatography. The peptides were subjected to automated Edman degradation in the presence of Polybrene. The sequence showed no evidence of heterogeneity. Large interspecies similarities, as well as amino acid interchanges, are apparent among various C lambda regions; the degrees of homology between rabbit C lambda region and the C lambda region from human, porcine and murine chains are 72.6, 71 and 72% respectively. The similarity of lambda-chains of different species is much greater than that found between rabbit kappa- and lambda-chains (about 34% homology). Genetic implications of these results are discussed.