The internal viscosity of human red blood cell membranes was investigated during exposure to continuous wave 1.0-GHz microwave radiation using fluorescence measurements of a lipid seeking molecular probe, diphenylhexatriene. Samples were exposed in a Crowford cell arranged so that fluorescence was measured during microwave exposure; specific absorption rates calculated from electrical measurements were approximately 0.6, 2 and 15W/kg. Measurements were obtained at selected temperatures between 15C and 40C and as a function of the duration of exposure at 23C. Arrhenius-type plots of the temperature profile data were linear and showed no difference between exposed and control samples. The exposure duration data also showed no difference between exposed and control samples except for a small effect of elevated temperature at the highest exposure. The activation energy for motion of the fluorescent probe in its environment within the membrane lipid was not affected by the application of the microwave energy and no evidence for a lipid phase transition was found. These results indicate that the increased cation efflux from red cells, observed by others at certain transition temperatures during microwave exposure, was more likely to have been caused by alteration of the membrane bound protein than by changes in the lipid constituents of the red cell membrane.