Chimeras established by transferring spleen or lymph node cells from primed donors use donor-derived B lymphocytes in responses to the antigen used in priming of the donor, whereas cells of the recipient are used in responses to other antigens. Clonal dominance by donor cells lasts for at least 9 mo. Treatment of newborn rabbits with the antigen used in priming the donors elicits copious production of antibody bearing the donor''s allotypic markers in chimeras, but tolerance is induced in nonchimeric controls. Lasting and effective memory is also established in chimeras in the absence of immediate antigenic stimulation. Priming of the donor facilitates the specific, effective and enduring acquisition of immunocompetence in the recipient for the antigen used in priming the donor.