In the variant rabbit strain BASILEA, immunoglobulins G were shown to contain two distinct populations of IgG molecules whose light (L) chains belonged to the known A isotype and to a new K-like type. These two L chains differed from each other by electrophoretic, chemical, and antigenic properties. The K-like L chain fraction showed (i) an acid-labile Asp-Pro bond at the end of the joining region and (ii) a tryptic peptide, whose amino acid sequence ofthe NH2-terminal 15 residues was identical to the homologous constant (C) region sequence of b9 K chain with the exception of the residue in position 70, which is asparagine in the K-like chain instead of the characteristic half-cystine residue in all L chains of KB type expressing b4, b5, b6, or b9 allotypes. Serological and chemical studies have demonstrated to date the existence of five polymorphic forms of the domestic rabbit K light (L) chain allotypes designated b4, b5, b6, b9, and b4var (1-6). Extensive chemical differences in the constant (C) regions of b4, b5, b6, and b9 molecules have been reported (7-9) and are thought to reflect the expression of the antigenic determinants of the b allotypes (10). In standard domestic rabbits, A-type L chains are the minor component of the immunoglobulin L chain pool (11). However, this situation is reversed in a variant rabbit strain designated BASILEA, which was developed and maintained by Kelus and Weiss (12). Homozygous BASILEA rabbits are characterized by the lack ofK chains bearing b4, b5, b6, or b9 allotypic markers, which are substituted by L chains of the A type bearing C7 and C21 allotypic specificities (12-14). The primary structure of the constant region ofA L chains derived from an antibody IgG raised in BASILEA rabbits was recently completed and showed extensive homology with A chains of other species (14,15