Helper T cells specific for sheep erythrocytes were generated in lethally irradiated mice. The helper effect exerted by these cells was determined in vitro in spleen cell cultures. Irradiation of the helper cells in vivo or in vitro with 1-9 Gy resulted in a bimodal relation between radiation dose and helper effect when measured either immediately or 6 hours after irradiation. The dose-effect curve became nearly linear when tested 20 hours after irradiation. These results indicated that the helper population consisted of about 80 per cent of cells inactivated shortly after irradiation, possibly due to interphase death, and a second type showing a more retarded cell death. The latter cells were able to exert a helper function during the interval between irradiation and cell death.