Effects of ultraviolet irradiation of chloroplasts on several photochemical reactions mediated by Photosystem II were studied at room temperature. The Hill activity and fluorescence of variable yield were decreased by ultraviolet irradiation in parallel. The activity of 2,6-dichlorophenolindophenol (DCIP) photoreduction in irradiated chloroplasts was only slightly recovered by addition of diphenylcarbazide, an electron donor for Photosystem II. No restoration of the original fluorescence yield was observed on addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) or preincubating the irradiated chloroplasts with dithionite. Photobleaching of C-550 was much more resistant than the Hill activity or fluorescence of variable yield toward ultraviolet irradiation. Chloroplasts, of which the Hill activity and variable fluorescence had mostly been eliminated by ultraviolet irradiation, still showed C-550 photobleaching, which was in magnitude more than 50% of that in the original unirradiated chloroplasts. C-550 was shown not to respond to membrane potential. The photobleaching of C-550 is sensitized by Photosystem II and proceeds with unaltered quantum efficiency in ultraviolet-irradiated chloroplasts. These observations are interpreted to indicate that C-550 is not identical with Q, the hypothetical quencher of fluorescence, and that C-550 is a better indicator for the primary photoact in Photosystem II than Q.