In the alcoholic extract prepared from leaves and stems of Coleus rhenaltianus, two substances were detected which in the Avena-coleoptile-section test showed auxin activity. These compounds fluoresced blue in ultra-violet light. One of the compounds could be identified with trans-caflcic acid. The other one is a related compound, giving the same absorption spectrum and colour reactions. The physiological activity of trans-caffeic acid is discussed in connection with the activity of /nwu-cinnamic acid.