Earlier investigations had demonstrated that steryl-glucosides may stimulate growth in the Avena coleoptile bioassay (Kimura et al., 1975). As it was now possible to obtain synthetic substances in larger amounts, the effects of sitosteryl-, campesteryl-, stigmasteryl- and cholesteryl-glucosides were tested over a wide range of concentrations on the growth of Avena-coleoptiles, on the germination of cress seeds (Lepidium sativum L.) and on the H+-efflux of protoplast suspensions prepared from leaves of Vicia faba L. Avena-assay: concentrations from 0.4 to 200 ppm of steryl-glucoside were tested, in some cases with the addition of 1 ppm IAA. Fig. 2 shows that the dose response curves were mostly biphasic and that the 4 substances tested yielded similar results. Addition of IAA did not markedly change the shape of the curves in relation to the behavior of the control. To explain such dose response curves it is necessary to postulate a multisite system at the site of action, which might be the outer cell membrane. Germination: the percentage of germination was not markedly affected by steryl-glucosides applied in the same concentrations as in the Avena-assay. However the lengths of shoots and roots of 3 days old cress seedlings revealed dose response curves similar to those obtained in the Avena coleoptile test (Fig. 4). Protoplasts: if the mode of action of steryl-glucosides is similar to that of IAA, it must be possible to demonstrate a H+-efflux at the cell membrane. Therefore steryl-glucosides (40 ppm) were applied to a protoplast suspension containing 0.35 M D-mannitol and pH changes were recorded continuously. The results show that already 1.5 hours after application a marked drop in pH was detectable, whereas the pH of the control increased slightly over a period of approximately 7 hours. As the solutions appear to be slightly buffered, it must be born in mind that H+-efflux begins before the first changes can be measured. Addition of 10−4 M IAA accelerates the efflux of H+, whereas 10−4 M 2,4-dinitrophenol applied 1 hour prior to the application of the steryl-glucosides inhibits acidification (Fig. 6). As sterols are probably involved in the structure of biological membranes, it is concluded that steryl-glucosides and IAA may act in a similar manner at the outer cell membrane, inducing changes in permeability and hydrogen-ion secretion. It may be that steryl-glucosides or sterols themselves are a kind of secondary messenger affected by hormones, for example by IAA.