['The herbicidal mode of action of flamprop-M-methyl [methyl N-benzoyl-N-(3-chloro-4-fluorophenyl)-D-alaninate] was investigated.', 'For initial characterization, a series of bioassays was used, which indicated a mode of action similar to that of mitotic disrupter herbicides. Cytochemical fluorescence studies, which included monoclonal antibodies against polymerized tubulin, were applied to elucidate effects on mitosis and microtubule assembly in maize roots. When seedlings were root treated with 50 microM of flamprop-M-methyl, cell division activity in meristematic root tip cells ceased within 4 h. The compound severely disturbed the orientation of spindle and phragmoblast microtubules, leading to defective spindle and phragmoblast structures. Cortical microtubules were only slightly affected. In late anaphase and early telophase cells, phragmoblast microtubules were disorganized in multiple arrays that hampered regular cell plate deposition in cytokinesis. Microtubules of the spindle apparatus were found attached to chromosomal kinetochores, but did not show regular organization associated with a zone of microtubule-organizing centres at the opposite ends of the cell. On account of this loss of spindle organization, chromosomes remained in a condensed state of prometaphase or metaphase. Unlike known microtubule disrupter herbicides, flamprop-M-methyl and its biologically active metabolite flamprop did not inhibit soybean tubulin polymerization to microtubules in vitro at 50 microM. In contrast, soybean plants responded sensitively to the compounds.'] Does the herbicide flamprop-M-methyl have a new antimicrotubule mechanism of action?