Quantitative studies have been performed to determine the action of low-intensity visible monochromatic light on various cells (E. coli, yeasts, HeLa, Chinese hamster fibroblasts and human lymphocytes); also irradiation conditions (wavelength, dose and intensity) conducive to vital activity stimulation have been examined. Respiratory chain components are discussed as primary photoacceptors. The possible ways for photosignal transduction and amplification are discussed. It is proposed that enhanced wound healing due to irradiation with low-intensity visible laser light (He-Cd, He-Ne and semiconductor lasers) is due to the increasing proliferation of cells.