SummaryWe present a nondestructive, easy to handle, cheap, fast and efficient assay to score wheat responses to herbicides. The application of the assay is illustrated by the reactions of the bread wheat cultivars ‘Castan’ and ‘Recital’ to the urea her‐bicide, chlorotoluron. The results not only confirm the presence of a major tolerant allele controlling the differences in response between the two cultivars, but also, show the contributions of modifier genes present in ‘Castan’, ‘Recital’ and other