Samples of human blood plasma were irradiated with the laser beam (lamda=633 nm, P=13 mW, t=20 min) in vitro. In experimental and control (incubated in the dark) samples the complement hemolytic activity was measured. Laser irradiation had minor influence on the duration of lag-period and the rate of complement-dependent hemolysis via classical and alternative pathways; it also did not alter the functional activity of factor B, components C2 and C3. Nevertheless laser irradiation effect was seen in essential reduction of a bystander lysis coefficient. This new fact can elucidate some aspects of the clinical effectiveness of the blood laser irradiation.