The effect of low-powered He-Ne lasers (red beam, 632.8 nm, 10.2 mW, and green beam, 543.5 nm, 0.44 mW) on hypotonic hemolysis in human red cells was investigated. Red cell solutions (0.9% NaCl, pH 7.00) were centrifuged and the sediments were irradiated with the red or the green beam for 0 (control), 1, 3, 5, 7, 10, 15 and 30 minutes. Each of the 40 irradiated samples were transferred into hypotonic solution (0.405% NaCl, pH 7.00). After leaving the samples at room temperature for 10 minutes, optical density of every suspension was measured spectrophotomechanically at a wavelength of 545 nm to determine hemoglobin content. Hemolysis percentages of the control samples in hypotonic solution were 52.2 +/- 10.7% (mean +/- SD, N = 20) in the red group and 55.0 +/- 12.0% (N = 20) in the green group. Relative values of hemolysis in the hypotonic solution after irradiation with the red or the green beam decreased significantly between the 7- and 30-minute irradiation. These results demonstrate that low-powered He-Ne lasers have a protective effect on red cell membranes from hypotonic hemolysis and stabilize the cell membranes.