We performed an immunohistochemical analysis using a polyclonal antibody to determine the localization of CuZn-superoxide dismutase (SOD), a scavenger of superoxide anion radicals, in the human male genital organs. In the testis, intense immunoreactivity of CuZn-SOD was shown in both the cytoplasm and nucleus of the spermatogonia of the seminiferous tubules. Spermatocytes, further differentiated germ cells and Sertoli cells showed no or weak immuno-reactivity. In the ductus epididymis, the principal cells showed no or weak immunoreactivity except for the stereocilial region, while the basal cells showed relatively intense immuno-reactivity. In the ductus deferens, the prostate and the seminal vesicles, columnar and cuboidal epithelia showed CuZn-SOD immunoreactivity. The immunoreactivity was more intense in the epithelia of the ductus deferens than in the prostate or the seminal vesicles. Basal cells in the prostate also showed intense immunoreactivity. Collectively, the present immuno-histochemical results suggest that CuZn-SOD in the male genital organs is localized where it could play an important role in cell differentiation, including spermatogenesis. The CuZn-SOD could also play a role in local defence mechanisms against tissue damage mediated through superoxide anion radicals, as well as in providing SOD to the seminal plasma.