Studies of the regulatory action of animal steroid hormones have revealed specific receptor proteins present in higher concentration in the target tissues. The behavior of the hormone-receptor complex, subsequent to binding, has been clearly characterized in the uterine response to estradiol (1). Similar data have been obtained on the association between a metabolite of a-ecdvsone and the crustacean hepatopancreas receptor (2). Attempts to date have failed to demonstrate an analogous receptor system for gibberellin in plants, although in etiolated dwarf peas, labeled GA3 accumulated most in tissues that provided maximal growth response (3). Results from further fractionation of the hormone-enriched zones, with the objective of localizing the receptor site(s), were inconclusive (4). The specific radioactivity of the labeled gibberellin used in the studies with pea was at least three orders of magnitude lower than the values reported for the hormones used in the animal systems, and the low level of binding encountered in animal tissues indicates that low specific radioactivity was probably the basic limitation in the plant investigations.Nadeau and Rappaport (5) recently submitted methods for preparing and purifying tritiated GA1 of high specific radio. Growth, Preparation, and Treatment of Epicotyls. "Progress No. 9" dwarf peas (Asgrow Seed Co., 1972 harvest, lot no. 37055) were rinsed repeatedly with distilled water to remove the bulk of the captan (Orthocide) fungicidal dressing, and were then imbibed, with aeration, for 12 hr at laboratory temperature. The imbibed seeds were sown in trays containing moistened vermiculite, and were allowed to grow, without further watering, for 5 days, in an enclosed dark cabinet at 250. Then the epicotyls, which were 3-3.5 cm long, were cut just above the point of insertion into the cotyledon, and trimmed to a uniform length of 2.5 cm, measured from the apex of the hook. All operations to this point were carried out either in darkness or weak green light. Groups of 30 epicotyls were placed in 5-ml beakers each containing 1.0 ml of solution containing 58 nM labeled hormone; beakers were placed. in the dark or under red light. The entire volume of the solution was taken up during a subsequent 12-hr period.Preparation of Test Solutions. [1,2-3H Igibberellin Al ([3H]GA1), with a specific radioactivity of 43 Ci/mmole (2.6 X 108 dpm/ug) 1, was prepared and purified by the method of Nadeau and Rappaport (5), and dispensed from a stock solution of MeOH-EtOAc (1:1 v/v) containing 3.58 X 105 cpm/ /diter. Portions of this solution were reduced to drvness in a stream of nitrogen, and the hormone was then redissolved in the requisite amount of distilled water to provide a final concentration of 20 ng/ml (2 X 106 cpm/ml, 58 nM).The 16 Tissue samples in the weight range of 2.5-3.0 g were ground at 20 with a pestle and mortar in 1.0 ml of 10 mM Trismaleate buffer, pH 6.5, containing 2 mM reduced glutathione, per g of fresh weight. The fluid extracted from the resultant paste by squeezing thro...