The product of a new genetic locus closely linked to Igh-1 in the mouse has previously been shown to be expressed on T cells by using an in vitro fluorescent antibody assay. In vivo studies reported here show the determinant coded for by that locus is expressed in unimmunized animals on the precursor for a suppressor T cell regulating a primary response to SRBC. Antisera raised in allotype congenic animals by immunization with suppressor T cells (Ts) for the ARS.CRI stimulate Lyt2+3+ suppressor cells for unrelated antigens. The suppressive effects of the serum can be absorbed with T cells but not with B cells. Suppression of an IgM PFC response in allotype congenic animals confirms close linkage of the marker to Igh-1 but not to H-2. This determinant is expressed on Ts cells for many antigens and may represent a heavy chain linked "constant" portion of the antigen-specific receptor.