Kinetin at physiological concentrations causes significant reduction of GA 3 -promoted growth in excised Avena stem segments. Kinetin is therefore considered to be a gibberellin-antagonist in this system. A Lineweaver-Burke plot reveals that kinetin acts non-competitively with GA 3 . The kinetin inhibition of GA 3 -promoted growth can be seen within 6 hours. It was found that soluble protein is markedly increased by kinetin in the tissue during the first 3 hours, thus preceding the inhibition of GA 3 -promoted growth by several hours. At the cellular level, kinetin negated the blocking effect of GA 3 on cell division in the intercalary meristem portions of these segments. In fact, kinetin promotes both lateral and longitudinal cell divisions in intercalary meristem cells.