We previously showed that a chromosome 6 locus, IgK-Ef2, controls a pair of prominent bands in normal mouse light-chain isoelectric focusing profiles. Screening of myeloma light chains derived from BALB/c mice (an IgK-EF2 alpha strain) led to the identification of seven light chains cofocusing with the polymorphic bands controlled by IgK-Ef2. Complete sequencing of the variable (V) regions of four of the light chains indicates that they are all members of the same subgroup (Vk-1A) and they differ from one another by 1--3 substitutions. One of the protein differs from the prototype V-region sequence only in the deletion of a single residue at position 95 immediately preceding of J region. The other two differ from the protype V region by 3 (two framework [fr], one complementarity-determined [cdr]) and one (fr) residues, respectively. Complete V-region sequences of two closely related light chains derived from NZB mice (an IgK-Ef2b strain) indicate the NZB proteins are derived from a distinct Vk gene (Vk-1B), differing by four substitutions from the Vk-1A sequence. The results suggest that the IgK-Ef2 polymorphism may be a result of, at least in part, the loss of the gene(s) coding for the Vk-1A subgroups in IgK-Ef2b strains of mice. The nature of the sequence diversity found in the Vk-1A subgroup indicates that either it is coded by a repeated series of virtually identical genes or that somatic mutation of a single Vk-1A gene may give rise to substitutions in framework as well as cdr regions.