Mice were given whole-body irradiation (WBI) with 75 mGy X-rays which had previously been found to stimulate immunologic functions. This low dose radiation (LDR) potentiated [Ca~(2+)]_i mobilization in thymic and splenic lymphocytes in response to Con A and anti-CD3 McAb and activated protein kinase C in T and B lymphocytes of the spleen. The expression of TCR/CD3 molecules on the thymocytes was enchanced after LDR indicating an expedited maturation and differentiation process in the thymus. The changes in TCR/CD3 expression and [Ca~(2+)]_i mobilization in response to McAb-CD3 after LDR was found to be highly correlated. Meanwhile the transcription of c-fos and c-jun genes was up-regulated beginning 3 hours after LDR. The expression of IL2R in actived thymocytes was potentiated 24 hours after LDR which coincided with the previous finding of increased secretion of IL2 by splenocytes after wBI with 75 mGy X-rays. It is first reported in the present paper that LDR could stimulate immunologic functions throu Mice were given whole-body irradiation (WBI) with 75 mGy X-rays which had previously been to to stimulate immunologic functions. This low dose radiation (LDR) potentiated [Ca ~ (2 +)] i mobilization in thymic and splenic lymphocytes in response to Con A and anti-CD3 McAb and activated protein kinase C in T and B lymphocytes of the spleen. The expression of TCR / CD3 molecules on the thymocytes was enchanced after LDR indicating an expedited maturation and differentiation process in the thymus. The changes in TCR / CD3 expression and [Ca ~ (2 +)] _i mobilization in response to McAb-CD3 after LDR was found to be highly correlated. after LDR. The expression of IL2R in actived thymocytes was potentiated for 24 hours after LDR which coincided with the previous finding of increased secretion of IL2 by splenocytes after wBI with 75 mGy X-rays. It is first reported in the present paper that LDR could stimulate immunolo gic functions throu