An immunoassay was developed to detect IgE antibodies to the widely used antibacterial drug trimethoprim. Significant levels of trimethoprim-reactive IgE antibodies were found in the sera of two patients who had experienced life-threatening allergic reactions following administration of a combination of trimethoprim and sulphamethoxazole. No IgE antibodies reactive with sulphamethoxazole were found in the sera of either patient. Inhibition experiments revealed that a high degree of cross-reactivity occurs between the drug-reactive IgE antibodies and two structural analogues of trimethoprim, 6-hydroxy- and 6-chlorotrimethoprim. These experiments also indicated that the combining sites of the trimethoprim-reactive IgE antibodies in the two sera were probably complementary to different parts of the trimethoprim molecule. The assay should supplement skin testing in determining the offending drug in patients with suspected allergic sensitivity to trimethoprim-sulphamethoxazole complex.