Cell-free particulate preparations derived from sonicated, purified rat peritoneal mast cells can be radiolabeled with IgE. The IgE concentration-dependence and the kinetics of the binding reaction at pH 6.6 were very similar to those observed with intact mast cells. The number of IgE molecules bound per mast cell equivalent at saturation varied somewhat from one particulate preparation to the next and, on average, was about 50% of the binding capacity of the intact cells. Incubation of intact mast cells or of particulate fractions with low concentrations of IgE in serum-free media resulted in a decrease in the amount of bindable IgE which remained in the supernatants in excess over the amount of IgE which was bound to the cells. Specific binding of IgE to both particles and to intact cells at limiting IgE concentrations was stimulated up to approximately twofold by a variety of antibiotic, synthetic or high molecular weight (protein) protease inhibitors of which soybean trypsin inhibitor and p-nitrophenyl-p'-guanidinobenzoate were the most active. These substances also markedly protected the IgE, leaving more of it bindable to rat basophil leukemic cells in a second incubation. Binding to particulate fractions also occurred at pH 4.8 when bovine serum albumin was added to the incubations. The results are consistent with the view that normal peritoneal mast cells have a membrane-bound protease which has high selectivity for IgE.