Anti-Lyb 7 antibodies selectively inhibit in vitro immune responses to the TI-2 antigens but not to TI-1 antigens. The Lyb 7 differentiation antigen is distinct from Lyb 5 and is coded by genes linked to IgCH loci. Lyb 8 is a new B cell differentiation antigen defined by a monoclonal antibody. It is a molecule of 95 kd and is specified by genes linked to Gpi on chromosome 7. Since Lyb 8 is found on pre-B cells and since it has no demonstrable function in B cell activation, its current role is as an early marker that signifies commitment of stem cells to the B cell lineage. m-anti-Lyb 2 antibodies inhibit in vitro antibody responses to SRBC and TI-2 antigens but not to TI-1 antigens, m-anti-Lyb 2 induces B cell proliferation only in the Lyb 5+ subset of B cells. Lyb 2 antigens are suggested to play the role of a receptor for a physiological T cell-derived soluble mediator required at one stage of activation of B cells into antibody-forming cells.