Abstract: A study has been performed on the temperature dependence of the light exposure of human leukemic cells when treated with the photosensitizing agent δ-aminolevulinic acid, (ALA). The cells were incubated with 1mM ALA for 4h at different temperatures (31-45°C) and irradiated with different light doses in the blue spectral region (430nm). The dose-response curves were studied. Shoulders at the lower exposure regions were observed in the log-survival curves for incubation temperatures 31-37°C. The threshold dose values for the temperatures for which the shoulders had appeared were found to be 3.9, 3.2, 2.2, and 1.6min. for 31, 33, 35 and 37°C incubation temperatures, respectively. The phototoxic effect for the cells with these critical values was small. The activation energy for overcoming the shoulder reactions was 25kcal/mol. An activation energy of 9kcal/mol calculated for the cell type might be for the uptake of ALA by cells or protoporphyrin IX (PPIX) production in cells.