Investigation of initial stages of T-lymphocytes activation was performed by coherent phase microscopy (CPM) based on the measurements of phase thickness and diameter of the cell and its organelles. Lymphocytes where activated in two ways: by addition of 2 10-6 g/ml phytohemagglutinin (PHA) and with He-Ne laser (lambda = 633 nm, 10-3 W) radiation. It was shown that phase thickness is proportional to refractivity (difference between refractive indices of an object and the surrounding medium) and geometrical thickness. Phase thickness decreased during activation independently of the nature of the stimuli. Phase thickness in healthy donors was 220 + 60 nm; decreased to 110 + 30 nm 1 h after addition of PHA. The same values were achieved 30 min after stimulation with the He-Ne laser. Characteristic changes in phase thickness in the nucleus and nucleolus were observed after lymphocytes were stimulated with the laser.