AbstractRabbit kappa light (L) chain allotypes are controlled by the allelic genes b4, b5, b6, b9and a recently described allele b4v. The product of the b4vgene differs from that of b4by at least two amino acid substitutions in the constant region. A serologic distinction between the b4 and b4vallotypes has now been detected by a radioimmune assay using one of six anti‐b4 sera tested. The distinction was enhanced by fractionation of this antiserum on an immunoadsorbent containing cottontail rabbit IgG. IgG with L chains of the b4vtype are deficient in their ability to inhibit binding of b4 IgG to the antibody preparation. L chains isolated from b4 and b4vIgG samples could not be distinguished by this assay indicating that interaction with an H chain is required for expression of the L chain determinants that differentiate b4 from b4