1. Hemolysis is not the only manifestation of the photodynamic effect of eosine on red blood cells, fixation and precipitation of the hemolyzed cell constituents also occurring at proper concentrations of the dye. 2. Hemolysis, fixation, and precipitation of the cell constituents are brought about by non-irradiated eosine at proper hydrogen ion concentration and concentration of the dye. Irradiation shifts the region of occurrence of these phenomena into lower concentrations of the dye. 3. In unbuffered solutions of eosine the hydrogen ion concentration increased in the course of irradiation. 4. Hydrogen peroxide may produce hemolysis and fixation in proper concentrations. 5. Hydrogen peroxide and cosine may reënforce each other in producing hemolysis and fixation. 6. Photodynamic action may probably be regarded as a summation of the effects of the non-irradiated photodynamic substance together with the oxidative changes brought about by this substance due to irradiation. 7. Oxidative changes in cell constituents, and changes in hydrogen ion concentration, together with the change in solubility of cell constituents brought about by the photodynamic substance, may possibly account for all the phenomena generally classed under the terms photodynamic action or photodynamic sensitization.