In this article we consider the role of the cyclin-dependent protein kinase cdc2 in regulating progression through the fission yeast cell cycle. The onset of mitosis is governed by cdc2 in partnership with the B-type cyclin, cdc13. Recent evidence shows that the cdc2-cdc13 complex can also control the onset of S phase and, in addition, ensures that there is only one S phase per cell cycle. This leads us to propose a novel quantitative model in which different levels of cdc2 activity regulate cell-cycle progression: S phase is initiated when protein kinase activity increases from a very low to a moderate level; maintenance of this moderate level prevents re-initiation of S phase, and a further increase of activity to a high level initiates mitosis. Inactivation of the kinase activity at the end of mitosis resets the cell for a new cell cycle.