L-Tryptophan decarboxylase (TDC) (EC 4.2.1.27) enzyme activity was induced in cell suspension cultures of Catharanthus roseus after treatment with a Pythium aphanidermatum elicitor preparation. The enzyme was extracted from lyophilized cells containing high levels of TDC and the protein was purified to homogeneity. The pure protein was used to produce highly specific polyclonal antibodies, and an enzyme-linked immunosorbent assay (ELISA) was developed to quantitate the level of TDC antigen during seedling development and in leaves of the mature plant. Westem immunoblotting of proteins after SDS-PAGE with anti-TDC antibodies detected several immunoreactive proteins (40, 44, 54.8, 55, and 67 kilodaltons) which appeared at different stages during seedling development and in leaves of the mature plant. The major 54.8 and 55 kilodalton antigenic proteins in immunoblots appeared transiently between days 1 to 5 and 5 to 8 of seedling development, respectively. The 54.8 kilodalton protein was devoid of TDC enzyme activity, whereas the appearance of the 55 kilodalton protein coincided with the appearance of this decarboxylase activity. The minor immunoreactive proteins (40, 44, and 67 kilodaltons) appeared after day 5 of seedling development and in older leaves of the mature plant, and their relationship, if any, to TDC is presently unknown. Results suggest that the synthesis and degradation of TDC protein is highly regulated in Catharanthus roseus and that this regulation follows a preset developmental program.