When [¹⁴C]dl-N-malonyltryptophan (MTrp) was fed to hypocotyls and roots of five-day-old Chinese cabbage seedlings, three CH₂Cl₂-soluble metabolites were identified by HPLC as IAA, indole-3-acetaldoxime (IAOX), and indole-3-acetonitrile (IAN), respectively. The in vitro conversion of [¹⁴C]MTrp by plasma membranes from Chinese cabbage seedlings enriched by aqueous two phase partitioning showed formation of IAOX as the only non- polar reaction product. The conversion of MTrp to IAOX showed a broad pH optimum between 5 and 8. The K_m for MTrp at pH 8 was 6.25 μM, pH 5 6.6 μM. Simultaneous feeding of [¹⁴C] MTrp and [³H]tryptophan as well as inhibition experiments with [¹⁴C]tryptophan and unlabelled MTrp showed that both substrates were converted by the same enzyme system. The formation of IAOX was reduced under anaerobic conditions, suggesting that the reaction was O₂-dependent; no other cofactors were needed. Other peptide-bound tryptophan derivatives were not accepted as substrates. It was shown that the enzyme system accepts only l-tryptophan as substrate. The role of MTrp as an auxin precursor is discussed.