We compared apoptosis in mouse thymocytes following exposure to low doses of high linear energy transfer (LET), 62.5-MeV (p-->Be+) fast neutrons and low LET, 4-MeV photons by flow cytometric analysis of hypodiploid cells. The incidence of apoptotic cell death rose steeply at very low radiation doses reaching a plateau of 3 Gy. Both the time course and the radiation dose-response curves were similar for high and low LET radiation modalities. The relative biological effectiveness (RBE) of 1.0 for apoptosis in the mouse thymocyte system contrasts with the much higher value typically seen in many classical systems of clonogenic cell survival and tissue response. This difference suggests that while radiation-induced apoptosis may contribute significantly to loss of susceptible cells at doses of < or = 2 Gy, it appears to have a questionable role in determining the relative intrinsic radiosensitivity of mammalian cells to high and low LET irradiation at clinically relevant levels of cell kill.