In vitro immunomodulatory properties of gangliosides have been well characterized such as the ganglioside-induced modulation of CD4 on T lymphocytes and inhibition of lectin-induced proliferative response of lymphocytes. These findings have led to an interesting suggestion that gangliosides play a role as in vivo immune modulators, although this possibility is not clearly defined yet. We then first confirmed in vitro effects of gangliosides on murine immunocytes and examined in vivo effects of gangliosides on immune response in mice. Murine spleen cells that were treated with a ganglioside mixture (GS) purified from bovine brain exhibited a marked decrease in CD4 expression, while CD8 expression was slightly suppressed. Transplantation of GS-untreated control immunocytes that were isolated from syngeneic mice into the immune suppressed mice by X-ray irradiation restored in vivo immune responses, while GS-treated cells could not. Immune response was assayed by the evaluation of footpad swelling which was induced by immunization with sheep erythrocytes as antigens. Moreover, intramuscular administration of gangliosides into mice suppressed both immediate (Arthus)-type and delayed-type allergic reactions. These results suggest that gangliosides would be potential in vivo immune modulators.