- To detect and characterize products of chlorophyll degradation, chlorophylls were photobleached in methanol solutions and compared with those occurring in vivo upon photodamage to the cyanobacterium Anabaena variabilis. In chlorophyll a solutions, visible light induced the formation of photoproducts with an absorption peak at 410-420 nm and a broad long-wavelength absorption band extending to 750 nm. Several photoproducts with absorption peaks at 405 nm and higher polarity than chlorophyll were resolved by reversed-phase HPLC. In addition, some intermediate photoproducts that emit fluorescence at 620-630 nm were detected. Spectral changes in the visible region that appear during photooxidation of the total pigments extracted from leaves of Urtica dioica L. were mainly due to products formed from chlorophyll a. Pigment bleaching in isolated membranes and whole cells of A. variabilis under irradiation with visible or UV light was associated with the formation of an absorption peak at 410-420 nm, which could be also attributed to the products of chlorophyll a photodegradation. It is suggested that the products of chlorophyll oxidation in vivo (presumably linear tetrapyrroles and porphyrin-like compounds) can act as endogenous photosensitizers. The changes revealed in chlorophyll absorption spectra can serve as indicators of (photo)oxidative damage to plants.