The hematopoietic cell kinase (hck), a member of the src family of intracellular, membrane‐associated protein tyrosine kinases, is primarily expressed in mature granulocytes and monocyte/macrophages. Hck kinase activity plays an essential role in macrophage activation and may be an important signaling molecule in granulocytes. To examine the potential role of hck in hematopoietic differentiation pathways, retroviral vectors were used to express wild‐type and mutant forms of p59^hck in the hck‐negative, interleukin‐3 (IL‐3) ‐dependent murine myeloid cell line, 32Dcl3. Constitutive expression of an activated form of hck markedly prolonged the viability of 32Dcl3 cells in the absence of IL‐3 but failed to abrogate the requirement for IL‐3 for proliferation. Moreover, enforced expression of the activated hck kinase (and less so, the wild‐type kinase) blocked granulocytic differentiation of 32Dcl3 cells in response to granulocyte colony‐stimulating factor. These findings indicate that up‐regulation of hck expression is not required for (and may interfere with) granulocytic differentiation. J. Leukoc. Biol. 60: 667–673; 1996.