Normal mouse B lymphocytes were tested for the ability to cap plasma rnemhrane antigenantibody complexes following exposure to 2.45-GHz continuous wave (CW) microwaves at power densities up to 100 mW/cm2 (45 W/kg specific absorption rate), at 37, 41, and 42.5 "C. After a 30-minute treatment, the irradiated cells and the nonirradiated controls were tested for capping by the direct immunofluorescence technique. First, thc cells were incubated for nine minutes at 37 "C with fluorescein isothiocyanate-conjugated goat antimouse immunoglobulin. After fixing and washing, the percentage of capped cells was determined under a fluorescence microscope. The results show that for the nonirradiated controls, capping is reduced from 90% at 37 "C, to 52% at 41 "C, to less than 5% for cells that were pretreated at 42.5 "C. There was no significant difference between the microwave-treated cells and the controls when both werc maintained at the same temperature. In another experiment, there was no significant difference in the percentage of capping between controls and cells that were exposed to microwave radiation during capping, when the temperature in both preparations was kept at 38.5 "C. The results demonstrate that B-lymphocyte capping is sensitive to temperature in the range that is proposed for use in tumor therapy.