CBA/N mice have an X-linked B-cell maturation defect which is reflected in part in an absence or dysfunction of a subclass of mature B cells. We have immunized the defective male offspring of the mating (CBA/N female X BALB/c male) with BALB/c spleen cells. The resulting antiserum (alphaLyb3) selectively reacts with a component on the surface of a portion of B cells from a panel of H-2 different mouse strains. Binding of alphaLyb3 serum to this B-cell subclass results in substantial (10- to 20-fold) enhancement of the antibody response to low doses of SRBC. Both binding and enhancing activity are removed by absorption with B cells from B6 and BALB/c, but not CBA/N mice. Absorption of the serum with bone marrow cells, T cells, or thymocytes from Lyb3+ strains does not remove activity. Since the enhanced plaque-forming cell (PFC) responses are specific for the immunizing antigen, and since no PFC response is produced by injection of the antiserum alone, this enhancement probably reflects a second signal produced by specific interaction between antibody and the surface Lyb3 component. Moreover, this signal can partially replace the requirement for T cells in the production of antibody to a "thymus-dependent" antigen. These findings (taken in conjunction with the previously described immune defects in CBA/N mice and other studies of B-cell maturation) suggest to us that Lyb3 is a cell surface component expressed selectively on a mature B-cell subclass. This component is important in B-cell triggering by antigen and fails to develop in CBA/N mice, due to a dysfunction of a regulatory gene on the CBA/N X chromosome.