The present study characterizes four potentially informative polymorphic bands of 5.2, 2.3, 1.9, and 1.2 kb, detected by Southern blot hybridization of Eco RI digests of human DNA using HK101/80 (an immunoglobulin V kappa I probe). These restriction fragment length polymorphisms (RFLP) show Mendelian segregation and they are linked to each other and to Km(1), the allotypic marker on the kappa constant region. There is strong linkage disequilibrium between the 2.3 and 1.2 kb polymorphisms. A 0.7 kb Pst I polymorphic band and a 2.9 kb Sac I polymorphic band were also identified; the latter may reflect a region of tandem repeats in the V kappa region. No bands representing the alternative forms of any of the polymorphic restriction sites were identified. This implies either that genes are missing from the V kappa repertoire or that such bands are hidden because of comigration of fragments due to conservation of restriction sites. Evidence for comigration of fragments was obtained from independent V kappa clones and suggests that dark bands on Southern blots of Pst I digests must often represent several superimposed genes which have conserved restriction sites. The demonstration of RFLP within the V kappa region provides circumstantial evidence for polymorphic variation in the repertoire of V kappa structural genes. The RFLP reported here should be useful as genetic markers in future studies on the immune response and disease susceptibility in man.