The cell cycle is controlled by a number of highly conserved proteins, found in species as diverse as yeast and mammals. The study of these proteins is a rapidly advancing field that is increasing our understanding of normal and abnormal cell division. Disruption of the cell cycle has been demonstrated in several different types of neoplasm, and there is increasing evidence that, in head and neck tumours, there is aberrant control of cyclins, cell cycle protein kinases and their inhibitors. Because of the phase specificity of some of the control proteins, antibodies to them are proving to be of value in studying cell kinetics of both normal tissues and malignant tumours.