With pinacyanol as the supravital stain, a preferential effect on mitochondria of KB cells was achieved by the irradiation with the ruby laser beam. The observation confirmed the results of other workers using janus green B in the same experimental system. The preferential effect on mitochondria was noted in the area extending 8–10 µ beyond the nonpreferential damage of 4–5 µ in diameter. The opaque material associated with mitochondria possibly represented coagulated protein. The effect involved cristae mitochondriales without severe disarrangement of their structure. The opaque material could be interpreted as the result of direct interaction between mitochondria and the laser beam, even though the mitochondria were noted outside of the previously estimated focal spot size of about 3 µ Within the thickness of 2–4 µ of monolayered cells, larger areas of damage can be accounted for by divergence of the beam which is focused by a microscope objective of very short focal length. A threshold of biologic effectiveness is probably also involved.

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