Effect of Ca2+ on the conversion of 1-aminocycloprophane-1-carboxylic acid (ACC) to ethylene was studied with 2.5 day-old mung bean hypocotyl segments. The conversion of ACC in these tissues was inhibited by plasmolysis and sulfosuccinimidyl (hydroxyphenyl) propionate (sulfo-SHPP). The ACC induced ethylene production in HC (high calcium)-tissue grown on the Ca2+ added medium was greater than that in N (normal)-tissue. HC-tissue had a lower inhibition rate of ACC conversion by EGTA and Ca2+ -channel blockers than N-tissue. The rates of the ACC conversion by both kinds of tissues were stimulated by the Ca2+ ionophore A23187. From these results, we suggests a mechanism for the stimulative effect of Ca2+ on the conversion of ACC to ethylene as follows; in some tissues where ACC conversion is linked with plasma membrane, Ca2+ may be transported from apoplast through Ca2+ -channel into the cytoplasm ad stimulate ACC-oxidase activity.