Plants produce a peculiar group of natural products, particular to the plant kingdom, the secondary metabolites, which are very numerous and structurally diverse. Provided that plant cells can grow 'in vitro', their culture offers the possibility of producing some of these compounds of pharmaceutical interest in large quantities. Alkaloids, steroids, cardiotonic glycosides, quinones and terpens, for example. are produced by either cell suspension cultures or immobilized cells; sometimes at a higher rate than in the whole plant. These systems are also used to yield several substances by means of a given biotransformation reaction which cannot be achieved in any other way. The use of cell cultures in pharmaceutical industry is just one of the many sides of plant biotechnology, which is proving to become an indispensable technique soon in the future.