AbstractAt 10−4‐ 10−2mol/l a number of sulphonic acids and related compounds, especially indole (3) methane‐sulphonic acid and 2,3,6‐trichlorophenylmethanesulphonic acid were found to be active as growth substances in the pea test, Avena straight growth test and flax root test. These acids being completely dissociated at physiological pH values, the anion must be the active form.The relatively low structural specificity of the polar group in the side chain of active compounds is considered to be indicative for a physicochemical type