Cultured mesophyll protoplasts can be directly induced to follow different developmental pathways. In a medium containing auxins only, cells elongate and finally differentiate into very long tubular cells. NAA at a concentration of 1 mg/l was found to be the optimal trigger for this response. If both auxins and cytokinins are present in the culture medium, cell division is consequently induced. In the absence of auxins cells do not grow actively, even if cytokinins are present. During further culture the cells can always be switched from one developmental pathway to another by giving the adequate hormonal signal. In this way it is possible to generate sequences of alternating cell division and cell expansion activities in the population of protoplast-derived cells and their descendents. In the model system used, the cells respond homogeneously to the external hormone signals. This allows to conclude unambiguously that whatever the actual state of the cells, the auxin-only signal always induces expansion and differentiation and the auxin + cytokinin signal always induces division, eventually coupled to dedifferentiation.