The in vitro conversion of [¹⁴C]‐tryptophan to [¹⁴C]‐indole‐3‐acetaldoxime (IAOX) by microsomal membranes of Chinese cabbage (Brassica campestris ssp. pekinensis cv. Granat) has been studied. The reaction product was identified by thin‐layer chromatography (TLC) and high performance liquid chromatography (HPLC). Furthermore. IAOX was identified as an endogenous compound of Chinese cabbage by mass spectroscopy. The tryptophan‐oxidizing enzyme (TrpOxE) was characterized. MnCl₂ was required as cofactor, H₂O₂, and 2,4‐dichlorophenol (DCP) stimulated the reaction. The enzyme showed a pH optimum at pH 8–9 and a K_m for l‐tryptophan of 20 μM. The membranes containing TrpOxE activity were identified as plasma membranes by means of aqueous polymer two‐phase partitioning. The TrpOxE from Chinese cabbage was purified 3‐fold from plasma membranes by solubilization followed by (NH₄)₂SO₄‐fractionation, affinity‐chromatography with concanavalin A, and native gel electrophoresis. Enzyme activity was reduced by a tunicamycin pretreatment. Several other plant species, e.g. maize (Zea mays L. Inrakorn), sunflower (Helianthus annuus L. cv. Hohes Sonnengold), tobacco (Nicotiana tabacum L. cv. White Burley), and pea (Pisum sativum L. cv. Krombeck) showed a similar conversion of [¹⁴C]‐tryptophan to [¹⁴C]‐IAOX by phase‐partitioned plasma membranes.