A novel selection system for plant genetic transformation was developed based on the enzyme tryptophan decarboxylase (TDC; EC 188.8.131.52) from Catharanthus roseus. This enzyme converts the toxic tryptophan analogue 4-methyl tryptophan (4-mT) into the non-toxic compound 4-methyl tryptamine. Expression of tdc in transgenic plants that have no endogenous TDC-activity allows selection on 4-mT. A vector was constructed containing a tdc cDNA clone under control of the constitutively expressed cauliflower mosaic virus 35S promoter. This vector was used in Agrobacterium-mediated tobacco leaf disc transformation experiments. The optimal concentration for selection with 4-mT was found to be 0.1 mM. The transformed nature of shoots obtained after tdc gene transfer and subsequent selection on 0.1 mM 4-mT was confirmed by northern blot analysis.