Low-dose radiation (LDR) can potentiate cellular metabolic activities or immune functions in vivo (hormesis), and can render cells resistant to DNA or chromosome damage caused by subsequent high-dose radiation (adaptive response). Protein synthesis was required for these cellular responses to LDR. In the present study, the early expression of proteins by thymocytes in response to low-dose whole-body irradiation (LD-WBI) was investigated. The expression of novel and previously existing proteins was found in the nucleus, cytoplasm, and extracellular fluid of thymocytes at 4 hours after WBI with 75-mGy X-rays. A 10 kD protein (RIP10) was seen in the cytoplasm of thymocytes after LD-WBI was further investigated. The fraction containing RIP10 separated by Sephadex G 100 gel filtration potentiated spontaneous thymocyte, and mitogen-induced splenocyte proliferation. Western blotting demonstrated that an anti-RIP10 antibody could react with a 10-kD cytoplasm protein and also with a 13-kD nuclear protein in thymocytes at 4 h after LD-WBI. Immunocytochemical staining showed the existence of RIP10 in several immune tissues including thymus, spleen, and lymph node. RIP10 expression, as determined by immunocytochemical staining and flow cytometry, was enhanced at 4-8 h after LD-WBI. Cell-cycle arrest (G(0)/G(1) block with decreased percentage of S-phase cells), and increased levels of spontaneous or radiation-induced apoptosis were observed in thymocytes incubated with RIP10 antibody in vitro for 4 h or 24 h. These results directly demonstrated the role of RIP10 in modulating cell proliferation and apoptosis. This finding is important to understand the mechanisms underlying LDR-induced hormesis and adaptive response.