Studies were carried out on the neutrophil activity in a course of a ulcerative colitis (UC). Phagocytic ability of granulocytes was assessed in spontaneous and stimulated nitroblue tetrazolium (NBT) reduction test. The studies included 60 patients who had 82 relapses and 11 patients with remission. They were classified into mild, moderate and severe relapse according to severity of disease. Increased phagocytic ability of neutrophils in spontaneous and stimulated NBT reduction test was shown.