We studied the influence of UV-B radiation on chlorophyll and rubisco activation by rubisco activase in the leaves of jackbean (Canavalia ensiformis). Chlorophyll content was decreased, indicating that the synthesis of those molecules may have been degraded or repressed after exposure. Rubisco content was significantly lower in radiated tissue compared with the untreated control; rubisco activity showed a similar pattern of change. Based on these data, we suggest that rubisco activity is associated with the level of rubisco protein, and that UV-B inhibits its activation and induction, as well as that of rubisco activase. Therefore, we propose that the inhibitory effect of rubisco by UV-B may be caused by rubisco activase.