The heavy chain variable region (V(H) marker J606-GAC, which is expressed on a subset of mouse heavy chain variable region group III antibodies, is expressed at similar frequencies on antibodies with μ, γ3, γ1, γ2, and α heavy chains. We have previously shown the J606-GAC determinant to be present on all anti-inulin and on the majority of anti-group-A-carbohydrate (GAC) antibodies examined. The responses to these two antigens are designated thymus-independent type 2 (TI-2) and thymus-dependent type 2 (TD-2), respectively, and have been shown previously to be largely restricted to the μ and γ3 heavy chain classes. TI-2 and TD-2 antigens are distinguished from other antigens such as T-independent type I (TI-I) and other thymus-dependent (TD) antigens, in part because they are virtually not immunogenic in CBA/N mice which express the x-linked immunodefiency (xid) allele. Surprisingly, we found no difference in the percentage of J606-GAC determinant-bearing plasma cells in the spleens of xid vs normal mice.