The effect of visible light (400-600 nm) on Candida guilliermondii and Saccharomyces cerevisiae was studied, and irradiation fluences killing the yeast cells without exogenous sensitizers were determined. The lethal effects are strongly oxygen-dependent, suggesting the involvement of photodynamic reactions mediated by endogenous sensitizer(s). Repair-deficient strains of S. cerevisiae show the same photosensitivity as the wild-type strain indicating that visible light does not photosensitize repairable DNA lesions. As was demonstrated using the microfluorometric method with the fluorochrome primulin, photodestruction of plasma membrane permeability barriers is important for yeast cell lethality. Visible light at cell-killing fluences induces lipid peroxidation in plasma membrane ghosts isolated from C. guilliermondii. Data obtained suggest the important role of singlet oxygen photogenerated by endogenous sensitizer(s) in initiating oxidative reactions. A spectrofluorometric analysis of the plasma membrane ghosts revealed one compound fluorescent in the visible spectral region at 683 nm. Its fluorescence excitation and absorption spectra have structures typical for porphyrins. The plasma membrane-bound porphyrin-like compound is different in some fluorescence properties from mitochondrial porphyrins. Estimation of its amount gives a value of 0.1 nmol porphyrin per milligram of protein of the plasma membrane ghosts. This porphyrin-type compound is considered to be the most probable candidate for the role of the sensitizer in photodynamic damage to yeast plasma membrane and cell inactivation by visible light.