Chlorophyllous cells of the liverwort, Marchantia paleacea var. diptera, contained three superoxide dismutase (SOD, EC 1.15.1.1) isozymes of Mn-, Fe-, and Cu/Zn-SOD. Cu/Zn-SOD was exclusively localized in cytosol. During one week of the culture, the Cu/Zn-SOD activity disappeared after 4 days, while Fe- and Mn-SOD activities were detected throughout the entire 7 days. An active Cu/Zn-SOD protein, judged from its active staining on a non-denaturing gel, was present for the first 3 days and disappeared after day 4, although the protein was present in 4- and 5-day-old cells determined on denaturing gel. An inactive Cu/Zn-SOD protein migrating faster than the active form appeared in 3- and 4-day-old cells on a non-denaturing gel but disappeared after day 5. This inactivation of the isozyme was suppressed by increasing the concentration of copper in the culture medium. The inactive protein was reactivated by the addition of copper in vitro. The active protein was converted to the inactive form by diethyldithiocarbamate, a copper chelator in vitro. These results indicate that conversion to the inactive form of Cu/Zn-SOD from the active form by release of copper is the first step in the process of its degradation.