The in vitro conversion of [¹⁴C]‐indole‐3‐acetaldoxime (IAOX) to [¹⁴C]‐indole‐3‐acetonitrile (IAN) by plasma membranes enriched by aqueous two‐phase partitioning of Chinese cabbage (Brassica campestris L. ssp. pekinensis cv. Granat) has been studied. The reaction product was identified by thin‐layer chromatography (TLC) and high performance liquid chromatography (HPLC). A reducing agent, e.g. ascorbic acid, was needed as cofactor for the formation of IAN from IAOX. Reduction equivalents and metal ions were not involved in the conversion of IAOX to IAN. The pH optimum for the reaction was at 6.0 and the apparent K_m for IAOX was 6.3 μM. The enzyme was not inhibited by thiol reagents. The pI of the enzyme was determined to be 7.1 by isoelectric focusing (IEF). Gel permeation chromatography showed one major activity peak of 40 kDa. The reaction is considered as part of a channeling process leading from tryptophan to IAN with IAOX as an intermediate. This process is probably regulated by the indole derivatives IAOX and IAN.