The action on plant growth of sixty benzotriazole derivatives carrying on position 1 or 2 several kinds of substituents, has been studied via the oat coleoptile section elongation test. At medium and low concentrations almost all compounds behave as activators, while at the highest concentrations many of them, like indolylacetic acid (IAA), are growth inhibitors. It is worth noting the lack of toxicity, even at the highest concentration used, for the 1-hydroxyalkylbenzotriazoles and, more generally speaking, the low toxicity of hydroxyalkylbenzotriazoles and of benzotriazolylalkanoic acid amides. In the range of concentration from 10(-6) to 10(-8) M some methylalkylbenzotriazoles as well as some benzotriazolylalkanoic acids and esters are more active than IAA exhibiting from 40 to 60% of the maximal activity of IAA. The activities of methylalkylbenzotriazoles and particularly that of 2-(1'-methyl)pentylbenzotriazole are outstanding; actually at the lowest concentration tested (10(-8) M and at 10(-5) M, at which concentration this compound shows the maximum activity, the biological responses correspond respectively to more than 50 and 80% of maximal activity of IAA.