An enzyme‐linked immunosorbent assay (ELISA) using a monoclonal antibody for the detection of ethyl parathion has been developed. The assay is capable of detecting ethyl parathion in water or milk in the range 0.001–40 μg ml‐1. The specificity of the technique was studied by inhibition assay. Cross‐reactivity with related compounds showed that the antibody reacted with ethyl parathion, methyl parathion and reduced parathion but did not react significantly with the structurally related compounds paraxon, p‐nitrophenol, malathion and dimethoate. Cross‐reactions at 40 μg gl‐1, the highest concentration assayed, produced inhibition by paraxon (40%), p‐nitrophenol (30%), malathion (15%) and dimethoate (0%).