We tested the hypothesis that patients who experience immediate hypersensitivity reactions to sulfonamides (SM) express IgE that can bind to a N4-sulfonamidoyl determinant (N4-SM). Sulfamethoxazole (SMX) was coupled to CNBr-activated cellulose disks to form a matrix predominantly substituted with isourea-linked N4-SMX determinants. After incubation of human sera with these disks or bovine serum albumin substituted disks as a control, the binding of IgE was assessed with 125I-labeled antihuman IgE. The binding ratios (counts per minute SMX disks per counts per minute bovine serum albumin disks) for sera from nonallergic donors and newborn infants averaged 1.11 (+/- 0.21 SD). Sera from 10 patients with histories of apparent immediate hypersensitivity reactions to SM were studied. Ratios greater than or equal to 2.1 (greater than 4 SD above control) were detected in 70% (seven of 10). Significant binding was detected in the sera of three of seven patients with other forms of SM allergy. Preincubation with SMX (80 mmol/L) inhibited binding 7% to 35% in eight of the 10 positive sera tested. Binding of one highly reactive serum was significantly inhibited by SMX, sulfamethizole, and sulfamerazine, but not sulfanilic acid or trimethoprim. The results of this study suggest that N4-SM is a major determinant recognized by IgE to SM and that an in vitro assay capable of detecting IgE to SM has been developed.