Radioresistant T lymphocytes (RRTL) were derived from spleens of CBA mice after gamma-irradiation (2000 rad). RRTL comprise 2.2-3.5% of the total T-cell population. Adult thymectomy and treatment with ATS or cortisol do not affect the yield of RRTL. The Thy-1 antigen content on RRTL surfaces exceeds that on thymocytes and splenic T lymphocytes. 51Cr-labelled RRTL fail to home into lymph nodes: a considerable number of labelled RRTL persist in the blood. In an adoptive transfer system RRTL display poor helper activity. The adoptive response of normal B cells plus RRTL was reduced when RRTL donors were preimmunized with the same antigen. It is not clear if the unusual properties of RRTL were induced by irradiation or if they pre-existed
