An early or preimmune repertoire for anti-insulin B cells was examined using the T cell-independent (TI) response to insulin conjugated to Brucella abortus (BA-ins). mAbs from the BA-ins response reflect a repertoire present 7 to 10 days before the first Ab-forming cells (AFC) are detected in primary T cell-dependent (TD) responses to insulin. Although 4 of 6 BA-ins mAb express IgG2 isotype, evidence for somatic mutation is limited. A total of 9 of 12 V regions are identical with known V(H) or Vkappa genes, and consensus sequences suggest two other V genes may be in germ-line configuration. The relative avidities (50% inhibition of binding) of TI anti-insulins cover a broad range and are consistent with a germ-line anti-insulin repertoire that is functionally diverse. The V(H)s of 5 mAb are from two subsets of J558 genes (205.1 and 186.3) that dominate the B cell pool of adult mice and are different from the V(H)s used by anti-insulin mAb in primary TD responses. One IgM anti-insulin (mAb 301) uses V(H)-J606 and Vkappa1, and this mAb binds beta cells. Other TI mAb use either Vkappa5 or Vkappa19.3 genes and are similar to Vkappa genes used by anti-insulin mAb from TD responses. The data show that mutations in germ-line genes are not required for measurable insulin binding by monospecific mAb from adult mice. The recurrent use of Vkappa genes in both early (TI response) and late (TD responses) suggest that these structures are important in insulin binding.