Abstract Insulin-like growth factor-I (IGF-I) promotes cell division and prevents programmed cell death in hemopoietic progenitors. Human HL-60 promyeloid cells differentiate toward the granulocytic lineage when stimulated with retinoic acid (RA) in serum-containing medium. When deprived of serum, however, we found that these cells differentiate poorly in the presence of RA, as assessed by expression of the alpha subunit of the beta2 integrin heterodimer, CD11b/CD18. However, when IGF-I is added to RA-treated cells, the proportion of CD11b-positive cells increases to a level similar to that in RA-treated cells cultured in serum-containing medium. Cells treated with RA alone not only differentiate poorly but also undergo apoptosis, as assessed by flow cytometry using propidium iodide and HO33342. In serum-free medium, one-third of RA-treated cells become apoptotic compared with only 5% apoptotic cells in the absence of RA. However, addition of IGF-I to RA-treated cells prevents the appearance of this apoptotic population and increases phosphatidylinositol 3'-kinase (PI 3-kinase) activity by fivefold. Wortmannin, a PI 3-kinase inhibitor, potently decreases this IGF-I-induced lipid kinase activity, blocks the ability of IGF-I to prevent apoptosis, and inhibits IGF-I-enhanced CD11b expression. These data demonstrate that IGF-I acts on RA-treated progenitors to promote their differentiation along the granulocytic lineage. IGF-I acts by rescuing these cells from apoptotic cell death via a downstream pathway that is dependent upon PI 3-kinase.