Abscisic acid (ABA) applied to intact bean (Phaseolus vulgaris) leaves or to isolated leaf discs inhibits light-stimulated cell enlargement This effect may be obtained with 10−4 mol m−3 ABA, but is more significant at higher concentrations. The inhibition of disc expansion by ABA is greater for discs provided with an external supply of sucrose than for discs provided with KC1, and may be completely overcome by increasing the KC1 concentration externally to 50 mol m−3. Decreased growth rate of ABA-treated tissue is not correlated with loss of solutes from growing cells, but is correlated with a decrease in cell wall extensibility. ABA does not prevent light-stimulated acidification of the leaf surface, and stimulates the acidification of the external solution by leaf pieces. However, the capacity of the cell walls to undergo acid-induced wall loosening is diminished by ABA-treatment. The possibility that ABA acts directly by inhibiting growth processes at the cellular level, or indirectly by causing stomatal closure, is discussed.