Effects of polysaccharide fraction from Euonymus alatus Sieb(EPF) on the immune response of T-, B-lymphocytes and macrophages were examined in vitro and in vivo system. EPF (500 mg/kg) were administered p.o. twice a day for 5 days to C57BL/6 mice, and then the cells were separated from mice. EPF decreased the viability of thymocytes, but increased the viability of splenocytes in vitro and in vivo system. Also, the administration of EPF enhanced the population of helper T cell and cytotoxic T cell in thymocytes and did not affect the population of splenocytes. Furthermore, EPF enhanced the phagocytic activity and the production of nitric oxide in peritoneal macro phages in vivo system. These results suggest that EPF regulates an immune response via the enhancement of mature T lymphocytes and B lymphocytes viability and phagocytic activity of macrophages.