By growing cultures of Rhizopus suinus Nielsen on filter-paper in large Petri dishes through which a sterile nutrient medium was slowly passed, the plant-growth hormone was accumulated in the medium. The function of the hormone is to allow increase in cell size in plant tissues, and its amount was determined as follows: the solution was mixed with an equal volume of 3% agar and from the mixture twelve equal blocks of volume 10.7 mm3 were cut; these were applied by means of gelatin to one side of decapitated Avena coleoptiles under standard conditions of 25[degree] and 85-90% relative humidity. The unit of the hormone is defined as that amount which when present in Ice. results in an angle of curvature of 1[degree] when treated as above, 110 minutes being allowed for the curvature to develop. The technique is that of Went (Rec. trav. bot. neerl. 25, 1, 1928) and Dolk (Geotropie en Groeistof, Diss. Utrecht, 1930). Since the hormone was extractable with ether only from acid solutions it was deduced to be a weak acid. The partition coefficient between ether and water was first determined, and then the fractions which could be extracted with ether at different pH were found, the above method of assay being used. From the data the dissociation constant of the acid was calculated to be 1.8.10"5, i.e., similar to that of acetic acid. The activity of the hormone was completely destroyed by KMnO4 or H2O2, largely destroyed by warming with AgNO3 or normal HCl, and unaffected by hydroxylamine or normal NaOH. It was partially purified by extracting with ether and chloroform at different pH, the highest purity reached being equal to 0.0014 mgms. per unit of activity, i.e., only 7.10-6 mgms. actually on the plant would suffice to produce a curvature of 1[degree].