The expression and activity of NADPH oxidase increase when HL-60 cells are induced into terminally differentiated cells. However, the function of NADPH oxidase in differentiation is not well elucidated. With 150–500 μM H2O2 inducing differentiation of HL-60 cells, we measured phagocytosis of latex beads and investigated cell electrophoresis. Two inhibitors of NADPH oxidase, DPI (diphenyleneiodonium) and APO (apocynin), blocked the differentiation potential of cells induced by 200 μM H2O2. However, H2O2 stimulated the generation of intracellular superoxide (O2•−), which decreased in the presence of the two inhibitors. DPI also inhibited H2O2-induced ERK (extracellular-signal-regulated kinase) activation, as detected by Western blotting. Furthermore, PD98059, the inhibitor of the ERK pathway, inhibited the differentiation of HL-60 cells induced by H2O2. This shows that H2O2 can activate NADPH oxidase, leading to O2•− production, followed by ERK activation and ultimately resulting in the differentiation of HL-60 cells. The data indicate that NADPH oxidase is an important cell signal regulating cell differentiation.