The authors analysed the course and the results of examination of peripheral blood in dynamics and study of red cell stereo-ultrastructure in dynamics before and after sessions of intravascular laser irradiation of blood (ILIB) and treatment in 120 patients with peritonitis, in 60 of them ILIB was applied in the complex of intensive therapy (main group). The control group was made up of 60 patients. The studies showed a favorable curative effect of ILIB: the morphological characteristics of red cells and peripheral blood in patients with peritonitis improves. The laser irradiated blood evidently stimulates the bone marrow and the organs in which blood elements are stored as a result of which their ejection into circulation intensifies. The favorable effect of ILIB is also manifested by increase of the number of intermediate and reduction of the number of prehemolytic and degenerating forms of red cells in various stages of peritonitis, i.e. ILIB affects mainly the retransformation of stomatocytes into discocytes but produces no marked effect on echinocytic transformation.