Primary and secondary proliferative responses to hapten (TNP)-modified autologous cells as well as to allogeneic cells, antigen (purified protein derivative) and mitogens were used for in vitro studies of the radiosensitivity of human T cells. To be able to compare data obtained from the various assay systems, optimal responding/stimulating cell ratios were used and the post-irradiation period kept constant. With these precautions, no difference in radiosensitivity of the various T cell responses could be found whether they were induced by mitogen, antigen, allogeneic cells or TNP-modified autologous cells. Neither could, on a per-cell basis, any difference in radiosensitivity between primed and non-primed cells be detected. At 400 rad, about 10% of the response was recovered in either case. The immunocompetence of the T cells surviving irradiation, as estimated by the ability to give a TNP-specific response, was not impaired.