A further characterization of human extracellular superoxide dismutase is reported. The study was especially aimed at the interaction with substances known to interfere with CuZn superoxide dismutase and other superoxide dismutases. Extracellular superoxide dismutase is efficiently inhibited by cyanide and is about 3 times more sensitive than is human CuZn superoxide dismutase. The sensitivity to azide is much lower, but still about 3 times higher than that of human CuZn superoxide dismutase. Extracellular superoxide dismutase is about as rapidly inactivated by hydrogen peroxide as is CuZn superoxide dismutase. The sensitivity to diethyldithiocarbamate is very high and more than an order of magnitude larger than that of CuZn superoxide dismutase. Sodium dodecyl sulphate, under conditions suggested as being suitable for distinguishing between the insensitive CuZn superoxide dismutase and the sensitive Mn superoxide dismutase, efficiently inactivated extracellular superoxide dismutase. No antigenic similarities between extracellular superoxide dismutase and CuZn superoxide dismutase could be demonstrated. Anti-(extracellular superoxide dismutase) did not bind CuZn superoxide dismutase, and anti-(CuZn superoxide dismutase) did not bind extracellular superoxide dismutase.