To prepare monoclonal antibodies (mAbs) against the generic part of sulfonamides, a sulfathiazole derivative was chemically linked to carrier proteins in such a way that the aromatic amino group, common to all sulfonamides, was distal to the proteins. Four mice were immunized with the sulfathiazole-protein derivatives. The spleen cells of one of the mice were fused with myeloma cells to produce hybridomas of which the supernatants were screened in an indirect ELISA (iELISA) for the presence of sulfathiazole antibodies. After cloning, positive supernatants were tested in a competitive iELISA (ciELISA) for inhibition with 18 sulfonamides. This resulted in four different mAbs (all IgG1 kappa light chain) which recognized several sulfonamides. By use of the best monoclonal (27G3) and an optimized ciELISA protocol, eight structurally different sulfonamides showed 50% inhibition at concentrations less than 100 ngml-1 or 5 ng/well. However, other relevant sulfonamides (such as sulfadimidine, sulfatroxazole and sulfachloropyrazine) were detected at a high level only with this mAb. This means that the ciELISA (with the best Mab) showed a broad specificity for sulfonamides but the sensitivity towards the different sulfonamides varied too much to call it a generic sulfonamide ELISA.