It is first reported in the present paper that whole-body irradiation (WBI) with low dose x-rays could increase the expression of TCR/CD3 molecules on lymphocyte plasma membrane and potentiate the mobilization of [Ca,2+,],i, in response to anti-CD3 monoclonal antibody (McAb-CD3) and ConA in lymphocytes. Male Kunming mice were irradiated with 75 mGy X-rays at a dose rate of 12.5 mGy/min. At different time intervals after WBI cell suspensions were made from spleen and thymus. Fluo-3 AM was used as a fluorescent Ca probe to load the cells followed by fluorescence staining of the cells with L 3 T 4-PE/Ly 2-PE McAb's. Double fluorescence analysis with flow cytometry (FACScan,B-D) showed immediate increase in [Ca,2+,],i, in both the CD4,+, and CD8,+, subsets of the splenic and thymic lymphocytes in response to Con A stimulation, reaching a plateau within 5-10 min. It was found that the [Ca,2+,],i, mobilization process was potentiated 12 h after WBI with 75 mGy. The response of the two subsets was found to be essentially the same. Therefore, thymocytes were loaded with fluo-3 AM without fluoresceinated McAb staining to study the time course of the changes in [Ca,2+,],i, mobilization after WBI with 75 mGy. It was observed that from 4 to 24 h after irradiation the amplitude of [Ca,2+,],i, mobilization of the thymocytes in response to McAb-CD3 increased with time. In order to look into the cause of this phenomenon, the expression of both the TCR and CD3 molecules at the plasma membrane of the thymocytes was studied by staining the cells with McAb-TCR and McAb-CD3 followed by anti-mouse-IgG-FITC for flow cytometry. Interestingly, the expression of both the TCR and CD3 molecules on the thymocytes was found to increase in parallel and the amplitude of this increase was up-regulated with time from 4 to 24 h following WBI with 75 mGy. The changes in expression of the TCR and CD3 molecules as well as in [Ca,2+,],i, mobilization were highly correlated with r=0.970 to 0.993. It is known that the expression of the TCR/CD3 complex on thymocytes is a manifestation of their maturation. Therefore, it is assumed that low dose radiation could expedite the process of maturation in the thymus, thus potentiating its immunologic reactivity. The authors think that the enhanced expression of surface molecules, such as TCR/CD3, and facilitation of the signal transduction process, in which [Ca,2+,],i, mobilization occupies a crucial position, are important in the understandung of the molecular mechanism of immunoenhancement following low dose radiation.