The effects of insulin-like growth factor-1 (IGF-1) on highly enriched human umbilical cord CD34⁺ cells were investigated in vitro. CD34⁺ cells were cultured in serum-free medium containing stem cell factor (SCF), GM-CSF, and interleukin-3 (IL-3). Culture of CD34⁺ cells for one week in the presence of these cytokines resulted in a dose-dependent increase in total cell number. Addition of G-CSF together with SCF+IL-3+GM-CSF increased the proliferation of myelopoietic cells as determined by the number of cells expressing the myelomonocytic marker CD64 and the granulocytic marker CD15 without significantly altering the number of CD34⁺ cells in the cultures. In the presence of G-CSF, IGF-1 induced a dose-dependent increase in the total cell number and a moderate but significant increase in the percentages of CD15⁺, CD64⁺ cells with sustained CD34⁺ cell proliferation. We conclude that IGF-1 can enhance the in vitro proliferation of committed progenitor cells derived from umbilical cord CD34⁺ cells.