The compounds 3-(p-chlorophenyl-1,1-dimethylurea (monuron), 5-bromo-3-sec-butyl-6-methyluracil (bromacil), 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), 2-chloro-4,6-bis (ethylamino)-s-triazine (simazine), 1,1′-dimethyl-4,4′-dipyridylium salt (paraquat), and 3-amino-1,2,4-triazole (amitrole) were added to a complete nutrient medium on which chlorotic Nicotiana tobacum was grown in the dark. Herbicide concentrations generally were 0, 10-4, 10-5, 10-6, and 10-7 molar. Callus growth was inhibited by molar concentrations greater than 10-7 for the triazines, 10-5 for monuron, 10-4 for bromacil, 10-6 for paraquat and amitrole. At these concentrations, interference with a mechanism other than photosynthesis was responsible for inhibition of growth.