Cytokinin activity (soyabean callus assay) has been determined in excised apical buds of Dahlia variabilis before and after a period of 3 h with cut surfaces in contact with agar gel, in the agar gel and in xylem exudates from cut shoot stumps. Buds before diffusion gave three peaks of activity in the butanol fraction, one in the aqueous fraction, following paper chromatography. Two of the former diffused into agar gel, the third (in which most activity was recorded) decreased in level during the 3-h period but was absent from the agar diffusate. The water-soluble cytokinin remained at its original level and was absent from the agar diffusate. The three peaks of activity in the butanol fraction were also present in xylem exudates. Ether and ethyl acetate fractions contained callus-growth inhibitors which diffused into agar gel.