Continuing studies on the structure and properties of the dithiol and menadione-dependent p-nitrophenyl phosphatase of Clostridium sticklandii established that there are two cysteine residues per mole of protein. These presumably are maintained in the reduced form by the added dithiol during catalysis. A crude lipid fraction extracted from C. sticklandii cells with an isooctane-isopropanol mixture contained a substance(s) that replaced menadione as activator of the enzyme. This active material presumably is the natural quinone required for activity of the phosphatase and experiments are in progress to characterize it further.