We have examined the amino-terminal sequence of the kappa light chains of a set of monoclonal antibodies specific for one of the major antigenic determinants (Sb) on the influenza virus PR8[A/PR/8/34(H1N1)] hemagglutinin molecule. This set was believed to be structurally related from earlier serological analysis that typed these kappa chains as members of the variable (V) region V kappa 21 group [ Staudt , L. M. & Gerhard , W. (1983) J. Exp. Med. 157, 678-704]. Our sequence analysis confirms and extends this conclusion; all examples of this set belong to a subgroup of the V kappa 21 group, V kappa 21C . A special feature of this set of kappa light chains is that all examples were derived from the same mouse (designated H36 ). This sequence analysis along with the characterization of gene rearrangements at the kappa light chain loci of these hybridomas is consistent with the idea that certain members of this set are the progeny of one or two lymphocytes. Because of this potential clonal relationship, we can reach several conclusions about the diversity observed among these kappa light chains: (i) the diversity is due to somatic mutation, (ii) somatic mutations occur sequentially and accumulate in the first complementarity-determining region, and (iii) the extent of somatic variation in this sample is high, suggesting a somatic mutation rate of about 10(-3) per base pair per generation.