The effect of X irradiation on CD80 and CD86 expression in peritoneal macrophages of mice irradiated with whole body irradiation (WBI) and in J774A.1 mouse macrophage cells line irradiated was studied with immunohistochemistry (ABC method) . Results showed that the upregulation of CD80 expression induced by WBI and in vitro irradiation with 2Gy X rays appeared earlier than that with 0. 075Gy X rays, but no significant difference was found in the response of CD86 to these doses. The study on the dose—effect relationship showed that the expression of both CD80 and CD86 reached its peak at 0.075 Gy after WBI as well as in vitro irradiation with X—rays. The doses inducing the second peak of CD80 and CD86 upregula,¬, tion were higher in the case of WBI in comparison with the in vitro irradiation. The results suggested that the upregulation of expression of B7 molecules after irradiation may be an important factor involved in the enhancing effect of low dose radiation on immunity.