Nonirradiated mice of the W/WV genotype were injected with normal (+/+) bone marrow cells that had been treated with antiserum to Thy 1.2 and complement (C'). Such bone marrow cells had no effect on the number of macroscopic colonies formed in the spleens of these mice, but did not cure the anemia. The addition of +/+ thymocytes to these bone marrow cells restored their ability to cure the anemia in W/WV mice. These data suggest that a theat-sensitive cell is required in the promotion of differentiation of murine hematopoietic stem cells into erythrocytes, and that there is a deficiency of such a cell in the W/WV mouse.