The effect of Oxadin (a new Bulgarian antimicrobial chemotherapeutic agent) on some parameters of non-specific immune response was investigated in a rat model of infection. After mimicking natural Yersinia enterocolitica systemic infection the number and functional activity of blood leucocytes and peritoneal macrophages were compared between groups of animals treated with Oxadin before and after infection. A significant immunostimulating effect of Oxadin was found in both experimental groups but was better expressed when administered before Yersinia infection. Bactericidal response of peritoneal macrophages (killing ability) and phagocytic activity of polymorphonuclear leucocytes from animals treated with Oxadin and thereafter infected with Yersinia enterocolitica were significantly activated during the first week of study. These findings correlated with the enhanced number of both types of phagocytic cells and the higher glycolytic activity of peritoneal macrophages.