The ability to remove receptor-bound IgE but maintain functional integrity of human blood basophils allowed measurement of several parameters of binding of IgE. Non-specific binding was determined by a modification of the standard technique and was found to be only about 10% of specific binding. The number of free receptors varied inversely with the serum IgE of the donor. Forward rate constants were readily measured at 0 degree but were considerably higher at 37 degrees, and a rough estimate of the activation energy for binding was calculated to be 6700 calories/mol. Saturation of basophil receptors with IgE occurred in 1-3 h using 3.6 micrograms 125I-IgE/ml at 37 degrees. For seven donors, the forward rate constant, k1, ranged from 2.9 to 7.25 X 10(4) M/sec with a mean of 5.2 X 10(4)/M/sec. This is about one-third the reported value for cultured cord blood basophils. The backward rate constant, k-1, ranged from 1.4 to 3.9 X 10(-5)/sec with a mean of 2.5 X 10(-5)/sec. This is less than half that of cord blood basophils. The equilibrium association constant, Ka, ranged from 1.4 to 2.7 X 10(9)/M with a mean of 2.1 X 10(9)/M. This is comparable to the high affinity binding reported for cord blood and other basophils. There was no great difference in binding constants among the four atopic and three non-atopic donors studied. The affinity of monoclonal mouse 125I-IgE for IgE receptors on human basophils was about four-fold lower than that of human IgE. Sensitization of human basophils for histamine release with mouse IgE anti-DNP was confirmed. Non-specific human IgE but not IgG inhibited sensitization by mouse IgE, indicating that the same receptors were involved and that they were specific for IgE.