Purified malformin A1 (cyclo-D-Cys-D-Cys-L-VaI-D-Leu-L-Ile), a cyclicpentapeptide toxin from Aspergillus niger, was applied to the hypocotyl segments of mung bean (Vigna radiata L.) seedlings to investigate its role in regulating ethylene biosynthesis. Production of ethylene was induced by treating the plants with 0.1 mM indole-3-acetic acid (IAA). When 0.1 pM malformin A1 was then applied, ethylene production increased and the activities of two key enzymes for its biosynthesis, 1-aminocyclopropane-l-carboxylic acid (ACC)-synthase (ACS) and ACC-oxidase (ACO), were also stimulated. However, at levels of I or 10 laM malformin A1, both ethylene production and enzymatic activities were significantly reduced. In the case of ACO, in vitro activity was regulated by malformin A1, independent of ACS activity or the influence of IAA. Furthermore, the conjugate form of ACC, N-malonyl ACC, was significantly promoted by treatment with 0.1 I~M malformin A1. These data suggest that malformin A1 can modulate ethylene production through diverse paths and that its effect depends on the concentration of the treatment administered.