The effect of pure human insulin-like growth factor I (IGF I) on the colony formation of late stage erythroid precursor cells (CFU-e) from fetal mouse liver and adult bone marrow was studied in a serum-free culture system. We found that IGF I in physiological concentrations stimulated erythroid colony formation. The combined effect of IGF I and erythropoietin was smaller than the sum of their single effects. The number of colonies induced by IGF I was linearly dependent on the number of plated cells. Our results indicate that IGF I is the first clearly defined mitogen that stimulates the late stages of erythroid differentiation independently of erythropoietin.