An electron microscope study has been made of vitally stained single cells whose cytoplasm has been subjected to a localized ruby laser microbeam. Light and moderate laser absorption (the resultant of stain concentration and laser energy density) produced restricted selective damage of mitochondria in cells stained with Janus green B; heavy laser absorption resulted in mitochondrial damage, as well as in nonselective interaction with other cell structures. With four other basic vital stains, the polysomes, ergastoplasm, mitochondria and other organelles at the irradiated site were uniformly damaged. Unstained cells showed no morphological alterations. With light primary damage (that restricted to the irradiation site), no secondary effects of the incident radiation were observed. With moderate primary damage, however, secondary damage of the mitochondria in the unirradiated cell portions was produced, which was reversible within 4 hr after irradiation. Heavy primary lesions caused severe secondary alteration of all cell structures that was irreversible and cell death occurred within 2 hr. Surviving cells examined 24 hr after light and moderate irradiation could not be distinguished from unirradiated controls. The possible mechanisms involved in the production of laser-induced cellular alterations are discussed.