In previous experiments a correlation was found between the amount of IgE on human basophils and the IgE serum level. The results of these experiments are shown to fit a free exchange model with an approximately constant K-value. We investigated whether the free IgE-receptors on the basophils, which should be present according to this model, could be saturated by incubating the cell suspensions with IgE myeloma protein or with sera from patients with an elevated IgE serum level. Various incubation conditions were applied in sensitizing the leukocytes, and cells from both atopic and nonatopic subjects were used for testing, but no increase in basophil-bound IgE could be measured with the quantitative immunofluorescence technique. Nor could we demonstrate a significant dissociation of cell-bound IgE in a period of 24 h. Therefore another hypothesis is put forward, in which receptor turnover is taken into account: we assumed that that free receptors on the basophils had a shorter functional half-life, than the half-life of the IgE-receptor complexes. This model would explain the contradictory results mentioned above.