SummaryThe interaction of 125I‐human myeloma IgE with the human basophilic IgE receptor of normal and allergic individuals was quantitated by direct ligand binding analysis after removal of endogenous IgE. Using a mixed leucocyte preparation containing 2 to 5 basophils, the 125I‐IgE binding was specific for human IgE, paralleled the presence of basophils in the cell preparation, and was described by a time course similar to that reported for passive sensitization experiments. The Kdvalues were 4.0 to 13.2 nM for the normal individuals with 1.5 to 8.4 × 105receptors per basophil, while the IgE receptor of allergic individuals was described by Kdvalues of 4.2 to 11.7 nM and 3.0 to 4.7 × 105sites per basophil. Based upon the eleven individuals studied, these results suggest that the binding properties of the IgE receptor of normal and allergic individuals are si
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