Angelica gigas Nakai (danggui) is a popular herb which has been used as a blood-building decoction for recovery from weakness in the Chinese medicine. Its demand increased in functional foods and pharmaceutical industries. For its hygiene, fumigation has been used, but the use of fumigants are going to be prohibited for food processing. In order to investigate gamma irradiation technique for hygiene of danggui, the immunomodulation activity of danggui after irradiation was examined. The water extract of irradiated danggui showed a strong mitogenic effect on splenocytes in vitro to the same level of lipopolysaccharide (LPS) and phytohemagglutinin (PHA). The effect was not different from that of non-danggui. It was tested whether there was any difference between irradiated and non-irradiated danggui in effects on the secretion of antibodies and graft versus host reaction in vivo. It turned out that intraperitoneal (i.p.) administration of the extract of irradiated danggui for 4 days remarkably increased the number of antibody-secreting cells in mice injected with sheep red blood cells (SRBC). Splenomegaly, due to graft versus host reacton, was also increased after 7 days i.p. administration of the extract of danggui in mice injected with allogeneic splenocytes. In these two in vivo test, the effect were not different from those of non-irradiated danggui. These results indicated that immunomodulation activity of danggui might be preserved after irradiation. In the other experiments (data not shown), the irradiated danggui was stable in active component analysis and safe in genetic toxicity test. In further research, the stability in other physiological activity of irradiated danggui will have to be proved before practical application of irradiation for hygiene.