An instant elevation in intracellular Ca 2+ concentration ([Ca 2+] i) appears significant after the irradiation of cells with a He-Ne laser. Studies using various cell types have demonstrated changes in DNA fluorescence that were attributable to a process of the apoptosis. The apoptosis is activation of an endonuclease, which has relation to the intracellulle [Ca 2+] i. The study has discovered an increase of the apoptosis of the mice thymocytes following the irradiation time elongation with the He-Ne laser. The apoptosis shows an elevation of the subdiploid peak in a histogram.