SEVEN FIGURES
Yhotodynamic action occurs when organisms, tissues or cells are exposed to visible light in tlie presence of certain fluorescent dyes and oxygen. The net effect is one of injury to the biological system involving oxidation of essential cellnlar constituents. Many workers have attempted to ascertain the nature of tlie mechanism underlying photodynamic action without complete agreement, some claiming primarily superficial action of the dyes, others indicating action of tlie dyes deeper in the cell (see Bl~iiii, '41, for summary). An effort was niade in the experiments reported here to gain fnrtlier insight into the mechanism of photodynamic action by dyes of the fluorescein series.* Specifically the investigation sought to determine the effects of photodynamic action on (1) entry of dye into yeast, (2) some of the metabolic activities of yeast, hotli during and after irradiation and (3) reprodiiction of yeast.
MATERIALS A X D METHODS Yeast was chosen as the organism for this investigation because it is easy to obtain in large numbers under fairly well controlled cultural conditions, and because wit11 it endo-Supported in part 1) :fmids from the Rockefeller Foundation. 'Penetrating dyes such a s neutral red and methylene blue, in f a c t basic dyc~s generally, were not studied. It is possible that the mechanism of action of such dyes is different, especinlly the entry of the dye, and an analysis of their artion merits a separate study.