An improved immunoassay for chlorpyrifos‐methyl (Reldan) in grain has been developed, based on an immobilized polyclonal antibody. This assay had greater sensitivity (limit of detection of 0.02 ppm in grain, or 0.05–0.1 ppb in buffer), was less susceptible to interference from methanol (used to extract residue from the grain) and was of greater precision than the earlier monoclonal antibody assay (Skerrittet al.,1992b). The polyclonal antibody exhibited greater cross‐reaction with chlorpyrifos‐ethyl (not used as a grain protectant), but less with fenchlorphos and bromophos (used occasionally as grain protectants), and employed a more stable peroxidase conjugate than the monoclonal antibody assay. Good correlations were obtained between chlorpyrifos‐methyl residue levels in wheat grain determined by the improved immunoassay and by gas chromatogra‐phy. The properties of the polyclonal antibody should also allow its use in a rapid field assay.