Four haptens analogous to phenylurea herbicides were synthesized. Immunogens and coating proteins prepared by coupling these haptens to human serum albumin and ovalbumin, respectively. Antisera raised in rabbits immunized with three of the immunogens bound strongly to microtitration plates coated with homologous hapten, such that competitive assays were developed using heterologous coating proteins. The assay systems were characterized with respect to specificity to 13 related compounds and one selected for application to the determination of monolinuron, diuron, and linuron in foods. Satisfactory recrovery data were obtained by direct analysis of methanol extracts of foods fortified with 40–160 ng/g monolinuron or 0.25–2.0 μg/g diuron. Solvent partitioning and silicic acid column chromatography were only partially successful in removing interferences in the determination of linuron.