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Abstract.A series of 9-substituted derivatives of four common naturally occurring cytokinins was tested for growth response in the soybean callus assay. The 9-substituents, 2-carboxyethyl (-C2CO2H), 2-carbo-t-butoxyethyl (-C2CO2tBu), and 2-nitriloethyl (-C2CN), all reduced the biological activity of the parent compound. The order of activities was, in general, -C2CO2 H > -C2CO2tBu > -C2CN and followed the unsubstituted cytokinin response pattern ofThe nature of the dose-response curves indicate that the activities of the derivatives are a function of their affinities to a common receptor site and that their steric and polar properties determine that affinity.The 9-(2-carboxyethyl) cytokinins provide a practical alternate source of haptens for the raising of cytokinin antibodies, since sugar-cleavage oxidations are avoided in the preparation of the antigens. E-9-(2-carboxyethyl)-O-13-D-glucopyranosylzeatin (E-ZOG9C2CO2H) was synthesized for the purpose of making antibodies specific for E-O-13-D-glucopyranosylzeatin (E-ZOG) and was also assayed for its growth response.The results obtained with stable 9-substituted cytokinins are useful in defining guidelines for the design of ligands available to explore receptorsite topology.Structure-activity studies for small organic molecules have been very reWarding in a number of fieldsmmedicine, pesticide research, and the study of Reference to a company and/or product named by the Department is only for purposes of information and does not imply approval or recommendation of the product to the exclusion of others Which may also be suitable.