Dilute alkali extracts of leaves from size-controlling rootstocks inhibited the growth of stratified apple seeds. Correlation coefficients of −0.9140** and −0.7292* exist between the vigor of the rootstocks and the respective growth of apple and wheat seedlings in the leaf extracts. The crude extract was chromatographed in isopropanol:ammonia:water (10:1:1). Two zones of growth inhibition were detected using a watercress bioassay. These zones were at 0 to 0.1 and 0.4 to 0.5 Rf units. The amount of growth made by the watercress was in the same ratio as the known vigor of the rootstocks M.II, M.VII, and M.IX. Measurement of the rate of indoleacetic acid oxidation showed that the material at an Rf 0.4 to 0.6 affected IAA breakdown. The more dwarfing rootstocks stimulated the oxidation of IAA. The significance of the results are discussed in relation to the dwarfing of apple rootstocks.