The flk2 receptor tyrosine kinase has been implicated in hematopoietic development. Mice deficient in flk2 were generated. Mutants developed into healthy adults with normal mature hematopoietic populations. However, they possessed specific deficiencies in primitive B lymphoid progenitors. Bone marrow transplantation experiments revealed a further deficiency in T cell and myeloid reconstitution by mutant stem cells. Mice deficient for both c-kit and flk2 exhibited a more severe phenotype characterized by large overall decreases in hematopoietic cell numbers, further reductions in the relative frequencies of lymphoid progenitors, and a postnatal lethality. Taken together, the data suggest that flk2 plays a role both in multipotent stem cells and in lymphoid differentiation.