Differentiation-inducing factor-1 (DIF-1) is a putative morphogen that induces stalk cell formation in the cellular slime mold Dictyostelium discoideum. In this study, we have examined the effects of DIF-1 on the human leukemia HL-60 cells. DIF-1 at 10-40 microM suppressed cell growth in a dose-dependent manner, and approximately 50% growth inhibition was attained with 15-20 microM DIF-1. FACS analysis of cell-cycle phase distribution using propidium iodide revealed that many cells were accumulated in the G1 phase after treatment with 15-20 microM DIF-1. These concentrations of DIF-1 also raised [Ca2+]i in a dose-dependent manner irrespective of the presence of extracellular Ca2+, indicating that DIF-1 elicited Ca2+-release from some intracellular Ca2+ store(s). Most importantly, relatively low concentrations of DIF-1 (1-5 microM) were found to promote retinoic acid-induced cell differentiation. The present results indicate that DIF-1 may be a useful tool for the analysis of myeloid cell differentiation and have therapeutic potential in the treatment of human myeloid leukemia.