Mouse myeloid leukemia line cells, Ml, could be induced to differentiate in vitro into macrophages and granulocytes with ascitic fluid of animals bearing various tumors. Ml cells could not be induced to differentiate with dimethyl sulfoxide alone. During the culture of Ml cells, spontaneously appearing cells resistant to factors stimulating differentiation (D factor) in ascitic fluid were isolated. These resistant cells were more refractile to the toxic action of dimethyl sulfoxide than sensitive cells and grew in culture medium with 1% dimethyl sulfoxide. Although the resistant cells were not induced to differentiate with dimethyl sulfoxide alone, they were sensitized with the aid of dimethyl sulfoxide to undergo differentiation with the D factor in ascitic fluid.