The present study was designed to observe the effects of 5-HT on the immune functions of the splenocytes irradiated with 75 mGy X-rays in Kunmin male mice in vivo and in vitro. The results showed that 5-HT inhibited significantly the spotaneous proliferation of the normal splenocytes, but the effects of 5-HT on the responses of the normal splenocytes to ConA were not obvious. The responses of the splenocytes to ConA after X-irradiation with 75 mGy both in vivo and in vitro enhanced in certain concentration of 5-HT. The results suggest that 5-HT in certain amount may inhibit the splenocyte immune functions and participate in the immunoenhancement effect following low dose irradiation. But the effects of certain 5-TH concentrations on the proliferation of the splenocytes irradiated in vivo and in vitro were different. It increased in vivo, but inhibited in vitro. It is supposed that it may be related to the changes of the internal environment of the splenocytes.