Treatment of light-grownEuglenacells with acridine orange (AO) at non-lethal concentrations resulted in the permanent loss of their ability to form chloroplasts. However, dark-grown cells were insensitive to AO. Starvation of the cells under light culture or the addition of chloramphenicol (CM) deformed the chloroplasts and made them less AO sensitive. Cells with mature chloroplasts seemed to show the most sensitivity to AO. Illumination with intense visible light of the AO-treated cells, whether they had been light- or dark-grown, made a greater proportion of them liable to be bleached or to be killed. The photosensitization was evident over the range of wavelengths of 450–500 nm, where the absorption maximum of AO is located.When light-grown cells, after AO treatment, were held in darkness in a nongrowth medium for 24 hr, the bleaching effect was reduced. However, if held under a light of 620–670 nm, where cells were not photosensitized but were able to form chloroplasts, the AO-damage was not reversed. The possibility of repair after AO-treatment is discus
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