By means of a previously developed basophil staining and fixation technique, it was possible to identify human basophilic granulocytes in leukocyte suspensions, which had been subjected to an immunofluorescence technique with anti-IgE. Using fluoresceinated anti-IgE, the fluorescence intensity of the basophils was measured by means of microfluorometry as a reflection of the IgE-load per cell. The reproducibility of this technique and the influence of the incubation time were studied. The mean fluorescence intensity of the basophil varied considerably in six donors, but the cells of two atopic patients showed the highest intensity. IgE was eluted from the cells at pH 2.5 and measured in the supernatant. A correlation was found between the amount of eluted IgE per basophil and the mean fluorescence intensity of the basophils. The number of IgE molecules per basophil was found to be in the range of 15,000--500,000.