The last step in biosynthesis of the plant hormone ethylene, oxidation of 1-aminocyclopropane-1-carboxylic acid (ACC), is catalysed by the elusive ethylene-forming enzyme (EFE). EFE is induced by fungal elicitors in suspension-cultured tomato cells. We demonstrate that Xenopus laevis oocytes injected with RNA from elicitor-treated tomato cells gain the ability to convert ACC to ethylene. The enzyme expressed in the oocytes under the direction of plant RNA is indistinguishable from genuine plant EFE with regard to its saturation kinetics, its iron dependency and its stereospecificity to the diastereomeric ethyl derivatives of ACC, allocoronamic acid and coronamic acid. In tomato cells stimulated for different times with elicitor, the level of EFE correlates with the level of RNA directing EFE expression in oocytes. Hybridization and co-injection experiments demonstrate that the tomato RNA species directing EFE expression in oocytes are homologous to clone pTOM13 which has been shown to inhibit ethylene production in plants when expressed in antisense. Using a cDNA library from elicitor-stimulated tomato cells, we have isolated several homologues of pTOM13 and identified one of them, pHTOM5, as a clone of EFE on the basis of its functional expression in the Xenopus oocytes.