The in vitro survival of human peripheral blood lymphocytes and thymocytes was measured 4 days following graded doses of γ radiation. Results indicate considerable heterogeneity among lymphocyte subpopulations with respect to radiosensitivity. Total T lymphocytes were characterized by rosette formation with neuraminidase-treated sheep red blood cells (nSRBC); early T ( T E) cells, by early rosettes; and B cells, by their inability to form nSRBC rosettes. Late T (${\rm T}_{{\rm L}}$) cells were defined as${\rm T}-{\rm T}_{{\rm E}}$. Survival curves of T, T E, and B cells are biphasic. The radiosensitive and radioresistant components of T, T E, and B cells all have a D0 of about 50 and 550 rad, respectively. B cells appeared to be slightly more radiosensitive than T cells.${\rm T}_{{\rm L}}$ cells and thymocytes, however, appeared to be homogeneous with respect to radiosensitivity, both having D0 values of about 135 rad. The survival of T cells in mixed T and B cell cultures resembled that of separated T cells, suggesting that ionizing radiation has no significant effect on rosette formation. It also indicates that interactions of T and B cells do not significantly affect their radiation responses.

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