Leucocytes from human peripheral blood were acid eluted and sensitized in vitro with increasing concentrations of radiolabelled myeloma IgE. This sensitization step was performed with or without 30% IgE depleted serum. After the IgE binding, cells were washed and submitted to a challenge with monoclonal anti-IgE for the determination of the cellular sensitivity and reactivity in a histamine release assay. A sample of each of the sensitized cells was analyzed for its radioactivity and the number of basophils quantified, thus allowing the determination of the mean number of IgE molecules per basophil. Raising the IgE concentrations in the sensitization procedure led to an increase of the IgE on the basophil membrane, and to a concomitant elevation of the cell sensitivity. The presence of serum during the binding of IgE onto the cells lowers slightly the binding of IgE to the basophils but decreases strongly the cellular reactivity.