X-irradiation was used as a tool to investigate the radiosensitivity of different B and T precursor subpopulations as detected by three in vitro culture systems. The culture systems utilized in this study included antigen reactive cell assays (ARCA), polyclonal mitogen assays (PMA), and polyclonal effector cell assays (PECA). The order of radiosensitivity of these systems in both the B and T cell series was ARCA greater than PMA greater than PECA (D37 values for B cell responses: ARCA = 88.8 R, PMA = 125 R, PECA = 223 R; D37 values for T cell responses: ARCA = 160 R, PMA = 441 R and PECA = 1095 R). With all assay systems the B cell response was more radiosensitive than the T cell response. The extrapolation number (n) from the radiation survival curves was approximately 2.0 for T cell responses and approximately 1.0 for B cell responses. The value of 1.0 for B cell responses suggest that their extreme radiosensitivity may be due in part to a lack of repair mechanisms. These findings also suggest that the more primitive precursor cells are more apt to undergo cellular proliferation upon activation and that this event is more radiosensitive than is the final differentiation event of precursor cells into a progeny of functional end-stage cells.