A comparative study has been made of the photokilling of Sarcina lutea by 8-methoxypsoralen (8-MOP) and by toluidine blue. It has been found that photosensitization by 8-MOP differs from photosensitization by toluidine blue, in that it has a temperature coefficient of less than one, and that the presence of oxygen is not necessary for, and even is deleterious to, the photosensitization, the psoralen being destroyed by its presence. It has previously been shown that the presence of carotenoid

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... ments protects the cells of S. lutea from lethal p)hotosensitization by toluidine blue; it was found that the presence of these pigments has no protective effect in photosensitization with 8-MOP. Studies on the lethal photosensitization of S. lutea with toluidine blue suggested that the primary sensitive site of the photokilling was the protein of the cell membrane, as manifested by the destruction of membrane enzyme activity and the regulation of permeability. It has been found that photokilling by 8-MOP has no effect on these functions. A study was made on the effect of photokilling by 8-MOP on the production of penicillin-resistant mutants as an indication of an alteration in the cellular deoxyribonucleic acid (DNA) by the psoralen. Psoralen photosensitization resulted in the development of many penicillin-resistant mutants.