The biosynthesis of ethylene was examined in suspension-cultured cells of parsley (Petro- selinum hortense) treated with an elicitor from cell wails of Phytophthora megasperma. Untreated cells contained 50 nmol g-1 of the ethylene precursor, 1-aminocyclopropane-l-carboxylic acid (ACC), and produced ethylene at a rate of about 0.5 nmol g 1 h-1. Within 2 h after addition of elicitor to the culture medium, the cells started to produce more ethylene and accumulated more ACC. Exogenously added ACC did not increase the rate of ethylene production in control or elicitor-treated cells, indicating that the enzyme converting ACC to ethylene was limiting in both cases. The first enzyme in ethylene biosynthesis, ACC synthase, was very rapidly and transiently induced by the elicitor treatment. Its activity increased more than tenfold within 60 min. Density labelling with 2H20 showed that this increase was caused by the denovo synthesis of the enzyme protein. Cordycepin and actinomycin D did not affect the induction of ACC synthase, indicating that the synthesis of new m R N A was not required. The peak of ACCsynthase activity preceded the maximal phenylalanine ammonia-lyase (PAL) activity by several hours. Exogenously supplied ethylene or ACC did not induce PAL. However, anainoethoxyvinylglycine, an inhibitor of ACC synthase, suppressed the rise in ethylene production in elicitor-treated cells and partially inhibited the induction of PAL. Exogenously supplied ACC reversed this inhibition. It is concluded that induction of the ethylene biosynthetic pathway is a very early symptom of elici-