Despite much progress in chemistry and pharmacy, plant cells remain an important source of medicinal compounds. According to Farnsworth & Morris (Amer. J. Pharm. 148, 46-52, 1976) one quarter of all prescription drugs in the US contain substances, which must be isolated from plant material. For a number of reasons it has become difficult to obtain a sufficient supply of plant drug material in recent years. Plant cell cultures could offer a solution to this problem, as was recently demonstrated in Japan by Fujita et al. (in; Fujiwara, A, (Ed.): Plant Tissue Culture. Japan Ass, Plant Tissue Culture, Tokyo, pp. 399-400, 1982). Plant cell cultures are able to accumulate secondary compounds at high yield, which can also be produced by the differentiated plant (for a review see i.e. Alfermann, A. W. in: Tramper, J. et al. (Eds.): Biocatalysis in organic synthesis. Elsevier, Amsterdam: pp. 225-238,1985). Furthermore, there is an increasing number of reports describing new compounds found only in cell cultures but not in the differentiated plant. Such novel compounds may be produced after addition of an appropriate precursor or by direct biosynthesis of the cells. A further possibility would be to combine different biosynthetic pathways by fusion of protoplasts of different species. A successful approach to isolate cell lines producing novel compounds has been published recently by Kesselring (in; Pflanzliche Zellkulturen. Bundesministerium für Forschung und Technologie: Bonn: pp. 111-129,1985).