The proliferation and differentiation signals of myelogeneous U937 cells are provided by extracellular stimuli, such as lipopolysaccharide (LPS) and phorbol 12-myristate 13-acetate (PMA). In a DNA-native-polyacrylamide gel assay system, we demonstrated that a particular nuclease activity is expressed in PMA-stimulated U937 cells and secreted into the culture medium. The nuclease activity was induced in U937 cells by LPS treatment, while the secretion of the enzyme was undetected in the culture medium. Therefore, it is likely that the expression and secretion of the particular nuclease in U937 cells are controlled by extracellular stimulations, such as PMA and LPS treatment.