Regulation of hemoglobin synthesis depends in part on the population of cells available for erythroid differentiation. Mouse erythroleukemia cells were cloned, and the clones were induced with dimethyl sulfoxide to test the relative induction of β minor and β major synthesis. Cells of line 745 produced approximately 35 percent β minor after induction, and 39 clones of line 745 produced from 23 to 61 percent β minor. Further subcloning of the clone that produced 61 percent β minor led to three subclones, all of which produced more than 90 percent β minor. Thus one kind of hemoglobin regulation occurs at the cellular level.