Effect of water-soluble (polypeptides) fraction (PF) of Crithidia oncopelti on the chemiluminescence of human blood cells and murine splenocytes was studied. The study has shown that PF possessed a capacity to inhibit oxygen-dependent killer system of polymorphonuclear leucocytes in dose-dependent manner. This effect was of a complex nature. The inhibitory effect of PF was demonstrated also in respect to murine mononuclear phagocytes and splenocytes. It was suggested that, in this case, the suppressor effect was related with the influence on membranous cytokines.