Synchronous cell systems are highly desirable for investigating various aspects of plant cell biology. However, to date, the tobacco BY-2 cell line is the only plant cell line which can be synchronized to high levels. A cell synchrony starting from S phase is obtained after release of BY-2 cells from aphidicolin treatment, while that from M phase is available after release from a sequential treatment of aphidicolin followed by propyzamide. A high level of synchrony is only attained by using rapidly growing tobacco BY-2 cells that propagate ca. 100-fold in a week. Reduced levels of synchrony result if the growth rate becomes lower. Technical notes for maintaining the high growth rate of the tobacco BY-2 cell are described. Using this highly synchronous system it has been possible to demonstrate the cell cycle-dependent oscillation of many genes, such as cyclins, and characterize their role during the cell cycle. Furthermore, this system has facilitated the structural and biochemical analysis of cell cycle specific events such as the development of the phragmoplast and the formation of cortical microtubules. Other potential uses of this highly synchronized cells are also described.