The production of agglutinating antibodies to the Ea-1 erythrocytic antigens of Mus musculus appears to be under the control of a single Mendelian recessive factor. There was no evidence that the nonresponding strains, CBA/J and BALB/c, possess antigens capable of absorbing antisera to Ea-1a or Ea-1b red cells, arguing against the hypothesis that these antigens are not foreign to these strains. The locus controlling this response, tentatively designated Ir-2, is linked to the agouti locus in the fifth linkage group.