1-Aminocyclopropane-1-carboxylic acid (ACC) is known to be converted to ethylene and conjugated into N-malonyl-ACC in plant tissues. When .alpha.-amino[1-14C]isobutyric acid (AIB), a structural analog of ACC, was administered to mung bean (V. radiata L.) hypocotyl segments, it was metabolized to 14CO2 and conjugated to N-malonyl-AIB (MAIB). AIB inhibited the conversion of ACC to ethylene and also inhibited, to a lesser extent, N-malonylation of ACC and D-amino acids. Although the malonylation of AIB was strongly inhibited by ACC as well as by D-amino acids, the metabolism of AIB to CO2 was inhibited only by ACC, but not by D-amino acids. Inhibitors of ACC conversion to ethylene such as anaerobiosis, 2,4-dinitrophenol and Co2+, similarly inhibited the conversion of AIB to CO2. The malonylation of AIB to MAIB is intimately related to the malonylation of ACC and D-amino acids, whereas oxidative decarboxylation of AIB is related to the oxidative degradation of ACC to ethylene.