Previous studies indicated that cultured rat basophilic leukemia cells have surface receptors which bind IgE with high specificity. In this paper we describe some quantitative aspects of the phenomenon. The reaction mechanism appears to consist of a simple reversible binding reaction with k1 = 9.6 x 104 M–1 sec–1 and k–1 ⩽ 1.6 x 10–5 sec–1 at 37°C. The calculated KA is therefore ⩾ 6 x 109 M–1. The activation energy of binding was found to be 7.8 kcal/mol. The number of binding sites/cell varied between 3 x 105 to over 1 x 106. The binding was insensitive to pH's between 6–8 but at pH 3.0 complete dissociation of bound IgE occurred in ∼ 1 min at 0°C leaving the receptors for IgE intact. Ca++ plus EDTA and Mg++ plus EDTA produce a fairly marked reduction in binding capacity though these reagents alone produce much smaller effects.

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