Mice carrying 2 mutant W alleles usually have severe macrocytic anemias which result from deficiencies of hematopoietic stem cells (CFUs) (1). Anemic W39/W39 and W41/W41 homozygotes (2) have deficiencies in the numbers of femoral stem cells which correspond to the severities of their anemias. The non-anemic W44/W44 homozygote has as few stem cells as the W41/W41 mouse. Nevertheless, bone marrow implants from W44/W44 donors cure the anemias of W/Wv recipients while implants from anemic W39/W39 and W41/W41 donors do not. The peripheral hematologic differences between W41/W41 and W44/W44 homozygotes probably arise from qualitative differences intrinsic to their stem cells rather than from extrinsic hematopoietic factors. The hematopoietic environments of all 3 W homozygotes are relatively normal in that they support normal erythropoiesis when injected with congenic +/+ marrow. Even non-anemic W44/W44 recipients are repopulated with +/+ donor red cells, indicating that W44/W44 stem cells are at a disadvantage when competing with normal counterparts.