Leukocytes were passively sensitized, as judged by allergen-induced histamine release. Before and after passive sensitization, the amount of total IgE on basophil leukocytes was measured by quantitative immunofluorescence microscopy. No measurable increase in IgE load was observed. When leukocytes were incubated with acid buffer after passive sensitization, no allergen-specific IgE was found in acid eluates of these leukocytes. Preincubation of leukocytes with excess irrelevant IgE resulted in inhibition of a subsequent passive sensitization. Postincubation of in vivo- or in vitro-sensitized leukocytes with excess irrelevant IgE had no effect on the sensitivity of these cells towards allergen. When leukocytes were incubated with TNP-labeled myeloma IgE and subsequently with fluorescein-labeled anti-TNP antibodies, no fluorescence was observed on basophil leukocytes, although binding of TNP-labeled IgE was demonstrated by anti-TNP antiserum-induced histamine release. It is concluded that only small amounts of IgE become bound to basophil leukocytes during passive sensitization, compared with the amounts of IgE already present on these cells. Exchange of IgE between cells and sensitizing serum does not take place to a measurable extent during passive sensitization.