Five different haptens of the N-methylcarbamate insecticide metolcarb were designed and synthesized. All of the haptens were conjugated with ovalbumin (OVA) for the coating antigen, and one hapten containing all of the structure of metolcarb was conjugated with bovine serum albumin (BSA) for the immunogen. Two polyclonal antisera were raised against the BSA conjugate, and ten antibody/coating conjugate combinations were selected for studies of assay sensitivity and specificity for metolcarb. A class-specific combination was found, with the I50 of the assay ranged from 0.64 to 20.98 microg mL(-1) for seven tested N-methylcarbamate insecticides except for pirimicarb. Considering titer, I50 and cross-reactivity of all combinations of antibody/coating conjugate, a competitive indirect enzyme-linked immunosorbent assay (ELISA) in a homologous system, whose limit of detection (LoD) reached 1.4 ng mL(-1), was presented. The results of competitive ELISAs indicated that coating hapten structure can significantly affect not only assay sensitivity but also its specificity.