Radiation injury in defined populations of B cells was investigated utilizing an allotype congenic transfer system. The amount of donor immunoglobulin present in the recipient's serum was found to be directly proportional to the number of viable cells injected, and on this basis approximate D₃₇ values for the inoculated B cells were determined in several experiments and found to fall in the 70 to 145 rad range depending upon the specific experimental conditions. Since the sensitivity of T cells to radiation-induced interphase death can be modified by prior exposure to select mitogens and antigens, an attempt was made to demonstrate a similar phenomenon with respect to B cells. The results indicate that the radiosensitivity of B cells can be slightly influenced by prior incubation with mitogens and only equivocally changed by activation with antigen.