In contrast to rabbits of b4, b5, b6, and b9 allotypes whose serum immunoglobulins (Igs) are predominantly composed of kappa-type light chains, rabbits of the mutant Basilea strain have serum Igs that are largely of lambda type. We prepared several antisera that recognized a minor K2 (bas) light chain that is produced by Basilea rabbits. With these antisera we identified the K2 (bas) isotype in the serum of the original b9/b 9 male rabbit whose offspring displayed the Basilea mutant phenotype. It was present in one half of his nonmutant offspring which inherited b9 from him and another b allotype from their mothers. Breeding was conducted both in Basel and at the NIH to develop and maintain colonies of mutant Basilea strain rabbits. The data obtained during colony development confirm that the trait of expression of the bas allotype maps to the same genetic region (b locus) that is known to control the allelic b allotypes b4, b5, b6 and b9. Homozygotes or heterozygotes of b4, b5 or b6 allotype (bb/b b) were mated with homozygous bb~S/b has rabbits to produce Fls, and then Fzs as well as progeny of backcrosses to both homozygous parental types (bb/b b and bbaS/b ba~) were produced. The bas allotype segregates as an allele (or pseudoallele) at the b locus although there was a deficiency in recovery of homozygous bas offspring in both the F 2 and backcross matings to bba~/b b"~ parental type in the NIH colony. This selective deficiency may reflect a deleterious effect on survival of homozygous bas progeny.