The purpose of this experiment was to investigate both the immunomodulatory effect of evening primrose(EP) oil and the effects of EP oil on immunoregulation by cyclophosphamide in mice. EP oil at doses of 0.1, 0.2 and 0.4 ml/kg were orally administered to ICR male mice once daily for 28 consecutive days. Cyclophosphamide was injected intraperitoneally to ICR mice with a single dose of 5 mg/kg at 2 days before secondary immunization. Mice were sensitized and challenged with sheep red blood cells(S-RBC). Immnune responses were evaluated by humoral and cellular immune responses and non-specific immune response. The results of this study were summarized as follows; (1) The humoral immune responses such as hemagglutination titer(HA), hemolysin titer(HY), Arthus reaction and plaque forming cell(PFC) were significantly enhanced in the low dose EP oil administered groups(0.1 and 0.2 ml/kg). However, in the high dose EP oil administered group(0.4 ml/kg) the responses were significantly lowered. (2) In the case of cellular immune responses, delayed type hypersensitivity reaction(DTH) was significantly decreased in EP oil whereas rosette forming cell(RFC) was remarkably enhanced. (3) Activities of natural killer cells and phagocyte were generally enhanced in EP oil. In addition, serum albumin and globulin were also increased.