Active immunization with myelin basic protein (MBP) induces experimental allergic encephalomyelitis (EAE) in a variety of animal species, including rats and mice. We have previously described the ability of the newly described mouse lambda (λ) variable (V) region, Vλx, to confer MBP reactivity to an Ab. In this report, we have evaluated the heavy (H) chain isotype distribution of Vλx-bearing Abs in normal mouse serum. We demonstrate a biased H chain isotype association with Vλx light (L) chains with a skewing towards γ2a and 2b isotypes. The IgG2a restriction in normal mouse Igs is even more evident in Vλx-containing Abs that bind MBP. This was confirmed by the ability of purified polyclonal IgG2a Abs to bind MBP and the finding that most or all of the IgG2a Abs that bind MBP seem to harbor a Vλx L chain. The specificity of naturally-occurring Vλx-bearing Abs with MBP can be localized to a particular epitope encompassing residues 25-34 of the MBP molecule. Furthermore, virtually all of the reactivity of Vλx-containing Abs with MBP peptide 25-34 is associated with the γ2a isotype. Collectively, these results suggest that the interaction of Vλx with MBP seems to be facilitated by an association with γ2a which may reflect preferred V(H) usage by this isotype. Such unique pairing of particular H chains with Vλx L chains in Abs that bind MBP may be indicative of a new B-cell component involved in the pathogenesis of EAE.