The cell-bound IgE molecules on basophil granulocytes were enumerated by C1 fixation transfer technique. The average number of IgE molecules per basophil granulocyte was 10,000 to 40,000. The number did not have direct correlation with serum IgE level; however, a rough inverse relationship was observed between the number of cell-bound IgE molecules and the optimal concentration of anti-IgE for maximum histamine release from the cells. The results suggested that the number and affinity of cell-bound IgE antibody molecules probably determine the sensitivity of basophils to allergen. When leukocytes were incubated with 100 µg to 1 mg/ml of E myeloma protein, the number of basophil-bound IgE molecules significantly increased. The number of receptor sites for IgE molecules per basophils was estimated to be between 30,000 and 100,000. IgE molecules have high affinity for the receptor but the binding of the molecules with the receptor is reversible. The association constant of the binding reaction was estimated to be on the order of 108 to 109. Basophils from all normal individuals studied combined IgE molecules upon incubation with E myeloma protein. After sensitization with a reaginic serum, however, the leukocytes from some of these donors failed to release histamine upon challenge with antigen under the usual experimental condition. These cells previously termed “non-acceptor” cells, released histamine if they were challenged in the presence of D2O, which enhanced biochemical processes after the antigen-antibody reaction. The results indicated that even such “non-acceptor” cells combined IgE antibody upon passive sensitization with reaginic serum and, therefore, that failure of antigen-induced histamine release from these cells was not due to lack of IgE antibody on their surface.