The radioresistance of antibody-mediated immune response is lowest when radiation is given before antigen, and rises steadily with time after antigen stimulation to reach a maximum when serum antibody is at a maximum, suggesting that antibody-producing cells are highly resistant. Irradiation affects the antibody-mediated response by 1) lengthening the period between stimulation and production of detectabel levels of serum antibody, 2) decreasing the rate of increase in serum antibody levls, and 3) decreasing the maximum concentration of serum antibody. Irradiation affects the cell-mediated response by inhibiting immunization and delayed hypersensitivity reactions when administered before antigen. Because the cell-mediated response involves nonlymphoid cells whose precursors are also radiosensitive, irradiation following stimulation may also produce temporary suppression.