No cytokinin activity was found in methanolic mesocarp extracts after purification with petroleum ether and ethyl acetate. Cytokinin activity did appear, however, in aqueous fraction after acid hydrolysis or passage through Dowex 50 (H(+)) ion exchange columns. The level of this activity, the fruit growth rate, and the cell division rate were found to be positively correlated (with each other).An inhibitor to cytokinin activity in soybean callus bioassay was found in the petroleum ether fraction; this inhibitor is not abscisic acid. The activity of the inhibitor could be considerably reduced by raising the level of kinetin in the nutrient medium. The intensity of inhibition was found to be positively correlated with reduction in the rate of mesocarp growth.The reduction in fruit growth rate may be considered to be due to the diminution of bound cytokinin activity and (or) increased activity of an endogenous inhibitor.