Bone marrow chimeras were formed containing mixtures of DBA/2 (Fv-2ss, Hbbdd) and B10.D2 (Fv-2rr, Hbbss) bone marrow. When these mice were infected with the polycythemia-inducing strain of Friend virus, erythropoiesis was stimulated, but the proportion of B10.D2 hemoglobin fell rapidly and newly synthesized hemoglobin was essentially all of the DBA/2 type. The treatment of infected polycythemic chimeras with phenylhydrazine lowered the hematocrit and restored the synthesis of B10.D2 hemoglobin. These results imply that B10.D2 erythroid precursors are intrinsically resistant to Friend virus-stimulated erythropoiesis. The experiments also suggest that virus-stimulated erythropoiesis is not mediated by a factor or cell-cell interactions, unless such factors or interactions do not act across strain barriers.