Objective: To investigate the effect of low power density laser irradiation on cytoplasmic free calcium concentration and the change of plasma membrane Ca2+ - Mg2+ - ATPase activity of human peripheral blood lymphocytes. Methods: Human peripheral blood lymphocytes, in vitro, randomized into irradiated group or control group. Lymphocytes were exposed to low power density He-Ne laser for 15, 30, 60min, and then the cytoplasmic free calcium concentration in human peripheral blood lymphocytes were measured by fluorescent indicator Fura-2/ Am estimating technique and the change of plasma membrane membrane Ca2+ - Mg2+ - ATPase activity in lymphocytes were measured by Lowry's technique. Results: The concentration of intracellular plasmic free calcium was lower in irradiated group than in control group (P<0. 05) , and the activity of plasma membrane Ca2+ -Mg2+ -ATPase increased significantly. Those changes reduced along with the length of exposing time. It showed that the concentration of intracellular free calcium correlated with the change of plasma mem brane Ca 一M g 一ATPase activity．Conclusion!The acticity of plasm a m embrane Ca 一M g ～ ATPase was activated by low power density laser irradiation of certain energy，and then followed by the change of perm eability of plasma membrane，influencing the change of storage of intracellular free calcium and the change of permeability of cell membrane and also cellular function