This paper provides evidence that normal animal cells possess a unique regulatory mechanism to shift them between proliferative and quiescent states. Cells cease to increase in number under a diversity of suboptimal nutritional conditions, whereas a uniformity of metabolic changes follows these nutritional shifts. Evidence is given here that cells are put into the same quiescent state by each of these diverse blocks to proliferation and that cells escape at the same point in G(1) of the cell cycle when nutrition is restored. The name restriction point is proposed for the specific time in the cell cycle at which this critical release event occurs. The restriction point control is proposed to permit normal cells to retain viability by a shift to a minimal metabolism upon differentiation in vivo and in vitro when conditions are suboptimal for growth. Malignant cells are proposed to have lost their restriction point control. Hence, under very adverse conditions, as in the presence of antitumor agents, they stop randomly in their division cycle and die.