The activity of the ethylene-forming enzyme (EFE) in suspension-cultured tomato (Lycopersicon esculentum Mill.) cells was almost completely abolished within 10 min by 0.4 mM of the metal-chelating agent 1,10-phenanthroline. Subsequent addition of 0.4mM FeSO4 immediately reversed this inhibition. A partial reversion was also obtained with 0.6 mM CuSO4 and ZnSO4, probably as a consequence of the release of iron ions from the 1,10-phenanthroline complex. The inhibition was not reversed by Mn 2+ or Mg 2+. Tomato cells starved of iron exhibited a very low EFE activity. Addition of Fe 2 + to these cells caused a rapid recovery of EFE while Cu 2 +, Zn 2 + and other bivalent cations were ineffective. The recovery of EFE activity in iron-starved cells was insensitive to cycloheximide and therefore does not appear to require synthesis of new protein. The EFE activity in tomato cells was induced by an elicitor derived from yeast extract. Throughout the course of induction, EFE activity was blocked within 10-20 min by 1,10phenanthroline, and the induced level was equally rapidly restored after addition of iron. We conclude that iron is an essential cofactor for the conversion of l-aminocyclopropane-l-carboxylic acid to ethylene in vivo.