Exogenous ACC-induced ethylene production in mung bean hypocotyl segments was severely inhibited by the treatment of spermine in a 6 hr incubation. In protoplasts acquired from the same tissues and pretreated with ACC, the concentration of cytosolic Ca2+ was increased after the addition of exogenous Ca2+. However, this increase of Ca2+ concentration was strongly suppressed by spermine. In previous studies, an artifical increase of Ca2+ influx by the treatment of a Ca2+-ionophore stimulated ACC-induced ethylene production. The inhibitory effect of spermine on ACC-induced ethylene production was more prominent than that of putrescine which has fewer NH3+ groups than spermine. In addition, spermine more promintly suppressed ACC-induced ethylene production in protoplasts in which fewer Ca2+ ions were released from Ca2+-storage organelles. Also, the amount oftranscript of ACC-oxidase which converts ACC to ethylene was decreased by the treatemnt of spermine. However, this reduction resulted only through the suppressin of ethylene production in a 2 hr incubation of mung bean segments. On the basis of these results, we suggest that there isa coupling of ACC-uptake to the increase of cytosolic Ca2+ concentration. In addition, the reduction of exogenous ACC-induced ethylene production by spermine could have resulted, at least partially, from reducing the Ca2+ influx which stimulates ACC-oxidase activity.