The production of agglutinating antibodies to the Ea-1a erythrocytic alloantigen in Mus musculus has previously been shown to be controlled by a single Mendelian recessive factor when strain YBR was crossed with BALB or CBA. It has now been determined that when YBR is crossed with C57BL/10, responsiveness is dominant and a strong nongenetic effect occurs. When positive assortative matings were made between YBR-B10 F2 segregants, a significant response to selection occurred, and estimates of the heritability of the response could be obtained. After one additional generation of selection the high responder line included 81% responders while the low responder line included no responders.