Summary The effect of two auxin antagonists, 2,3,5-triiodobenzoic acid (TIBA) and 2-(p-chlorophenoxy)-2-methyl propionic acid (CMPA) on brassinosteroid (BR)-induced ethylene biosynthesis in etiolated mung bean hypocotyl ( Vigna radiata L. Rwilcz cv. Berken) segments was studied. The maximum inhibition of both ethylene and 1-aminocyclopropane-1-carboxylic acid (ACC) biosynthesis by TIBA and CMPA was achieved at concentrations of 50 μM and 10 μM, respectively. With increasing concentrations of BR, there were significant increases in both ethylene and ACC production. When varying concentrations of BR were used in combination with 50 μM TIBA or 10 μM CMPA, there was a decrease in ethylene and ACC production at all BR concentrations tested, with the exception of the lowest. After 18 h, mung bean hypocotyl segments pre-incubated for 8 h with either 50 μM TIBA or 10 μM CMPA showed a maximum inhibition of BR-induced ethylene and ACC production. The results of treatment sequences showed that both TIBA and CMP A caused the same inhibitory effect whether applied before or after BR-treatment.