This study investigated the influence of expression of proteins of the INK4 family, particularly p16, on the growth and self-renewal kinetics of hematopoietic cells. First, retrovirus-mediated gene transfer (RMGT) was used to restore p16^INK4aexpression in the p16^INK4a-deficient lymphoid and myeloid cell lines BV173 and K562, and it was confirmed that this inhibited their growth. Second, to sequester p16^INK4a and related INK4 proteins, cyclin-dependent kinase 4 (CDK4) was retrovirally transduced into normal human CD34⁺ bone marrow cells and then cultured in myeloid colony-forming cell (CFC) assays. The growth of CDK4-transduced colonies was more rapid; the cell-doubling time was reduced; and, upon replating, the colonies produced greater yields of secondary colonies than mock-untransduced controls. Third, colony formation was compared by marrow cells fromp16^INK4a−/− mice and wild-type mice. The results from p16^INK4a−/−marrow were similar to those from CDK4-transduced human CFCs, in terms of growth rate and replating ability, and were partially reversed by RMGT ofp16^INK4a. Lines of immature granulocytic cells were raised from 15 individual colonies grown from the marrow ofp16^INK4a−/−mice. These had a high colony-forming ability (15%) and replating efficiency (96.7%). The p16^INK4a−/−cell lines readily became growth factor–independent upon cytokine deprivation. Taken together, these results demonstrate that loss of INK4 proteins, in particular p16^INK4a, increases the growth rate of myeloid colonies in vitro and, more importantly, confers an increased ability for clonal expansion on hematopoietic progenitor cells.