AbstractA biphasic modulation of responsiveness of spleen lymphocytes to mitogens was observed in mice exposed to 2,450‐MHz radiation at power densities of 5–15 mW/cm2over various periods ranging between one and 17 days. This modulated phenomenon may be explained on the basis of 1) suppression of lymphocyte response by microwave‐activated macrophages which persists throughout the entire course of radiation, and 2) concurrent progressive direct stimulation of lymphocytes which culminates around day 9 of exposure. Tumor cytotoxicity of killer lymphocytes from mice exposed to five or nine days of radiation did not appear different from sham controls. The highly proliferative hematopoietic marrow cells were sensitive to microwave radiation. Nine days of exposure to radiation (15 mW/cm2) reduced the colonyforming units of myeloid and erythroid series by 50%. This observation may offer a new and more sensitive assay for studying biological effects of electromagnetic radi