Abstract Information about the IgE receptor and IgE content of mast cells under different conditions in vivo is essential for further understanding the functions of the mas cell-IgE system. A cytofluorometric method for measuring cell-bound IgE on individual mast cells was therefore explored using peritoneal mast cells of Sprague-Dawley rats infected with the nematode N. brasiliensis and rat basophilic leukaemia cells (RBL-1) as experimental models. We systematically studied the effects of variables such as fixation, incubation time, temperature and concentrations of antibody on the fluorescence emission of the mast cells. Optimum conditions were selected for the quantitative measurement of IgE at the single-cell level using direct labelling with FITC-conjugated goat anti-rat IgE(Fc). Polystyrene beads with a defined fluorophor content and a fluorescent uranyl glass were used to standardise the measurement procedure. A linear relatioship between fluorescence intensity and IgE concentration was obtained by fluorescence measurements on RBL-1 cells incubated in rat IgE. In the case of rat peritoneal mast cells it was possible to saturate the IgE receptors by incubating the mast cells in rat IgE. By measuring the mast cells before and after IgE incubation, the relative content of IgE, the relative number of IgE receptors and the degree of receptor saturation could be estimated. In this way we measured the IgE content of peritoneal mast cells of Sprague-Dawley rats maintained under pathogen-free conditions. The distribution of IgE content in the mast-cell populations was extremely variable. Up to 30% of the mast cells in individual populations contained little or no IgE, but very few if any of the cells lacked IgE receptors. On average, 60–70% of the receptors available for binding were occupied by IgE in these normal rats.