It has been suggested that the differential histamine-releasing activity of an IgE-dependent histamine-releasing factor (HRF), which has recently been cloned, is related to carbohydrate differences in the IgE molecule. Lectins are able to recognize specific glycoforms and might therefore be useful in characterizing the proposed heterogeneity of IgE molecules. As one test of this hypothesis, we examined the histamine release potency of several well-characterized lectins on basophils passively sensitized with serum containing IgE molecules that support HRF-induced histamine release (IgE⁺) or serum that does not support release by this stimulus (IgE⁻). Histamine release was induced by challenging basophils with different concentrations of concanavalin A, Lens culinaris (LcH), and Pisum sativum (PSA). Dose-response curves revealed that LcH caused 30% histamine release at 2 μg/ml with IgE⁺ sensitized cells, whereas the same release with IgE⁻ cells required sixfold higher concentrations. Similar values for PSA showed a sevenfold difference. With concanavalin A, the selectivity was reversed in that it was fourfold more active on IgE⁻-sensitized cells. Another pair of sera (IgE⁺ vs IgE⁻) revealed the same result for concanavalin A, but no difference occurred in LcH and PSA induced release between the IgE⁺- and IgE⁻-sensitized cells. These contrasting findings with different pairs of IgE⁺- and IgE⁻-containing sera indicated that the lectins LcH and PSA are not able to discriminate between IgE⁺- and IgE⁻-sensitized cells as does HRF and therefore cannot be used to further define the proposed heterogeneity of IgE. This conclusion was supported by experiments in which basophil preparations from donors possessing natural sensitivity or insensitivity to HRF (having IgE⁺ or IgE⁻ on their surface) were examined for their response to these lectins. No difference was found in the sensitivity of the cells to challenge with LcH or PSA, and the response to concanavalin A was the opposite of that found when passively sensitized cells were used (30% histamine release at 0.9 vs 3.5 μg/ml for IgE⁺ vs IgE⁻ donors, respectively). We conclude that oligosaccharide-specific lectins do not differentiate between HRF-reactive and HRF-nonreactive IgE molecules on basophils. (J ALLERGY CLIN IMMUNOL 1996;98:181-8.)