Immunoassays based on microwells (for laboratory assay) and polystyrene tubes (for field assay) have been applied to the analysis of the herbicides, molinate and diuron, in field water samples. Development of a new immunoassay format for molinate, with detection limits of 0.5–1 ppb, enabled, for the first time, the direct analysis of molinate in water samples across the full range of usual field residue levels, without need for pre‐concen‐tration. A highly sensitive field assay for urea herbicides (diuron limit of detection of 0.15 ppb) has also been developed, based on a double‐layer coating with protein A and urea herbicide‐specific antibody. Each assay was only slightly affected by ions and turbidity, which are often problems in subsurface and surface water matrices respectively. For both formats of the molinate and diuron immunoassays, close correlations were obtained between herbicide levels determined by the immunoassay and by instrumental analysis, using field water samples.