Rabbit antibodies prepared against two purified 7S BALB/c myeloma proteins, A2 (γ1, κ) and MOPC 195 (γ2b, κ), and solid-phase absorbed so as to contain only anti-heavy (H) chain specificities bound to 32 to 35% of C57BL/6 lymph node lymphocytes (about 80% of the B lymphocytes). A similar staining frequency was obtained with solid-phase absorbed antibodies to the Fab fragment and the heavy chain of the A2 protein. On the other hand, antibodies to other 7S proteins (MOPC 21 (γ1, κ), J606 (γ3, κ), and MOPC 173 (γ2a,κ)), as well as anti-Fc γ1, anti-Fc γ2a and anti-idiotypic antibodies either failed to bind to the cells, or bound to a small percentage of these. Employing a method of quantitative inhibition of cell-surface-binding capacity, it was found that the surface binding of the anti-A2 and anti-MOPC 195 antibodies was strongly inhibited by the MOPC 104E (µ, λ1) and J558 (α, λ1) proteins. There was also a reciprocal inhibition by the A2 and MOPC 195 proteins. Purified normal serum IgG1 and IgG2a were also strongly inhibitory. However, eight other BALB/c myeloma proteins of various classes were weakly, or noninhibitory. Only the homologous A2 Fab fragment, and H and light (L) chains inhibited the anti-A2 antibodies. The homologous and heterologous Fc fragments were not inhibitory. These results, added to the selective inhibitory behavior of the myeloma proteins, which is unrelated to either their H or L chain isotypic designation, suggest that the antigens involved are localized in H chain the variable region. The findings implicate that the bulk of C57BL/6 lymph node B lymphocytes expresses a surface H chain variable region subgroup antigen that corss-reacts with the A2, MOPC 195, MOPC 104E, and J558 myeloma proteins.