AbstractQuantitative structure‐activity relationship (QSAR) studies have been carried out on oral rat LD50values, for 129 herbicidal (trifluoromethyl)benzimidazoles, by multiple regression analyses, using computer‐generated descriptors to represent chemical structure, at several levels of specificity. The results permit rationalisation of the structure‐toxicity relationship, particularly the effect of substituent interactions, and allows reliable ‘rough estimate’ prediction of toxicity. However, reliable predictions of high accuracy are not feasible, even from analysis of this large, consistent, congeneric set of data, thus casting doubt on the suitability of the LD50as a biological end‐point