The immune response to polysaccharide antigens is highly regulated and has several distinguishing features including restricted subclass, variable region gene usage and fine specificity. The basis for selective VH and VL gene usage in response to antigens such as bacterial levan (BL) is poorly understood. BL is a beta (2->60 linked polyfructosan with beta (2->1) (inulin determinant) linked branch points. We have generated a panel of 102 monoclonal antibodies. (mAb) from BALB.cAnN and CBA/CAHN mice following one or two doses of 10 mcg BL. mAb from a single immunization are predominantly IgM and IgG3. VH gene analysis of CBA mAb showed a biased usage of J606 and 36-60 families with an expected frequency of J558 in response to single or multiple injections. In BALB/c Mab there is a pronounced VH restriction to the J606 family (84%) following a single injection of BL, but a more normal VH profile in response to two injections. VL usage is also restricted. There is a correlation in both strains (with only one exception in CBA) between VK11 usage and inulin reactivity of the Mab. Sequence data from three BALB/c J606/VK11 Mab show the use of one J606 gene, which has the same derived amino acid sequence as that of the J606 myeloma protein. 12 Mab have been generated from BL- injected CXBG/B4 mice which is a BALB/c x C57BL/6 recombinant inbred strain that expresses the BALB/c immunoglobulin heavy chain gene locus and the C57BL/6 Sr-1 gene, a diversity gene previously shown to influence fine specificity of the response to BL. CXBG/4 Mab are a mix of IgM, IgA, and IgG3, do not cross-react with inulin, and are currently being analyzed for V region gene usage. We conclude that CBA and BALB/c Mab specific for BL differ markedly in VH gene family usage and fine specificity, that light chain usage correlates with fine specificity, and that the BALB/c immunoglobulin heave chain locus is, itself, not enough to explain these differences. Molecular studies are in progress to further define the basis for these differences.