We developed an enzyme immunoassay for determining human cuprozinc superoxide dismutase (h-SOD) using two kinds of monoclonal antibodies prepared by immunizing h-SOD to BALB/c mice. This method was sensitive and specific enough to determine exogenous h-SOD injected into rats. When intravenously injected into rats, much of the immunoreactive h-SOD accumulated in the kidney and was rapidly excreted in the urine. We observed both a modified and an unmodified form of exogenous h-SOD in rat urine.