An expression plasmid has been constructed which includes DNA encoding human manganese superoxide dismutase. Such plasmids may be introduced into host cells and the resulting cells cultured or grown under suitable conditions so as to produce enzymatically active human manganese superoxide dismutase or analogs thereof which may then be recovered. A method of producing an enzymatically active analog of human manganese superoxide dismutase by supplementing the culture media with Mn.sup.++ has been developed.