A competitive enzyme-linked immunosorbent assay (ELISA) suitable for the determination of the urea herbicide isoproturon, 3-(4-isopropylphenyl)-1,1-dimethylurea, in food and environmental samples was developed. Two haptens named 1-(3-carboxypropyl)-3-(4-isopropylphenyl)-1-methylurea (hapten 4C) and 1-(5-carboxypentyl)-3-(4-isopropylphenyl)-1-methylurea (hapten 6C) were synthesized. The haptens were coupled to bovine serum albumin (BSA) and ovalbumin (OVA), respectively, using the N-hydroxysuccinimide reaction. The hapten 6C-BSA conjugate was used as the immunogen, with which a high-titer anti-isoproturon polyclonal antibody (pAb) was successfully obtained by immunization of New Zealand white rabbits. The hapten 4C-OVA conjugate was used as coating antigen and a method of the indirect competitive ELISA for isoproturon was established. The haptens were confirmed with TLC, IR, and 1H NMR. The conjugation molar ratios of hapten 4C to OVA and hapten 6C to BSA were 36:1 and 46:1, respectively, as calculated by a UV spectrophotometry. The highest titer of the anti-isoproturon sera determined by a non-competitive indirect ELISA procedure was 1.6 × 105. The optimal concentrations of the coating antigen and the dilution of the anti-isoproturon sera used in the ELISA were 0.1 mg L-1 and 1.0 × 105, respectively. The concentration of isoproturon that inhibits 50% of antibody-antigen binding (IC50) was 0.07 mg mL-1. The cross-reactivities of six urea herbicides including chlorbromuron, fluometuron, monolinuron were lower than 0.1%. Isoproturon is a small molecule without immune activity and active functional group for attaching to carrier protein. To produce an antibody against isoproturon with high titer and high specificity is the most important step in the development of an immunochemical method for the determination of isoproturon in food and environmental samples. The two haptens synthesized in this study have carboxyl groups and accommodate different lengths of spacer arms, and the phenyl and isopropyl groups are fully
