To count V kappa genes homologous to particular V kappa nucleotide sequences, thereby permitting estimates for the total V kappa repertoire, we hybridized 10 cloned V kappa cDNA sequences to restriction fragments of mouse embryo DNA (Southern blots). Particular probes labeled up to 17 fragments, of which up to 8 were strongly labeled. This indicates that the germline contains sets of related V kappa genes. Only 4 nonoverlapping gene sets of 16-22 genes each were found. Assuming that the probes used are a random sample of the V kappa pool, the pattern repetitions suggest that the germline contains a total of about 5 distinct V kappa gene sets and about 90 V kappa genes. Correlating sets of strongly labeled genes with V kappa groups having similar N-terminal sequences led to an estimate of about 300 germline V kappa genes, while extrapolation from the number of genes in the VK-21 group gave an estimate of 90-140 genes. Since the three independent estimates fell between 90 and 320 genes, the germline V kappa repertoire, at least as expressed in myelomas, probably lies in that range. The V kappa fragment patterns given by three probes did not differ detectably between BALB/c, NZB, and A/J DNA but all three patterns differed in AKR DNA, indicating that there is limited polymorphism within the mouse V kappa locus. Somatic mutation must expand the number of expressed V kappa sequences, since the 8-12 VK-21 genes estimated for BALB/c and for NZB is significantly less than the 22 known NZB VK-21 polypeptides.