Sulfa antibiotics (sulfonamides) are derivatives of p-aminobenzenesulfonamide that are widely used in veterinary medicine. Foods derived from treated animals may be contaminated with these drugs. However, current immunobased sulfonamide detection methods are unfit for screening of products because they are either too insensitive or specific for a few compounds only. An immunoassay capable of detecting all sulfas in a single reaction would be ideal for screening. For development of a binder capable of binding all sulfas, a protein engineering approach was chosen and the properties of monoclonal antibody 27G3 were improved with mutagenesis followed by selection with phage display. Several different mutant antibodies were isolated. The cross-reaction profile of the best mutant antibody was significantly improved over that of the wild-type antibody: it was capable of binding 9 of the tested 13 sulfonamides within a narrow concentration range and also bound the rest of the sulfas, albeit within a wider concentration range.