— Delayed photohemolysis of human erythrocytes sensitized with the fluorescein sensitizer phloxine B has been examined. A 0.05% hematocrit suspension of cells was irradiated in the presence of sensitizer up to 2 h with fluorescent lamps. Analysis for percent hemolysis was performed at various times afterwards by measuring absorbance due to hemoglobin in the centrifuged supernatant. Lysis develops along a sigmoid time course which varies with illumination period and sensitizer concentration. The hemolytic rate was found to vary with the square of the fluence. Changing the sensitizer concentration at constant fluence showed that lytic rate varied with the 1.4 power of concentration. These results were verified in separate experiments where both concentration and dose fluence varied.