Human peripheral blood lymphocytes isolated using a Ficoll-Uropolinum gradient were exposed to between 1 and 25 Gy X-rays and were cultured for period of up to 72 hours. Before irradiation and after 2, 24, 48 and 72 hours total numbers of surviving cells per ml of culture were determined as well as the frequencies of cells spontaneously forming rosettes (E) and active rosettes (AE) with sheep red blood cells, cells bearing receptor for activated complement (EAC) and cells spontaneously forming rosettes with mouse red blood cells (ME). Irrespective of the duration of in vitro culture, the radiosensitivity of the subpopulations of human lymphocytes increased in the order E less than EAC = ME less than AE rosette forming cells. Calculation of the frequencies of surviving cells, null with respect to the surface markers (E and EAC), showed that disappearence of rosette-forming cells under the influence of radiation was largely due to the shedding of their receptors and not exclusively to cell death. It is suggested that the extreme radiosensitivity of the subpopulation of T lymphocytes capable of AE rosette formation, if confirmed by in vivo studies, may possess diagnostic significance in persons accidentally exposed to radiation.