The products of the cdc13+ and cdc2+ genes form a stable complex that displays protein kinase activity in vitro. p63cdc13 is a substrate of p34cdc2, the catalytic subunit of the kinase. The histone H1 kinase activity of cdc2 oscillates during the cell cycle. Activation of the preformed cdc2/cdc13 complex at the G2/M transition requires cdc25+ gene function. Post-metaphase inactivation of the kinase is associated with loss of cdc13, which shares sequence homology with mitotic cyclins and, in common with these proteins, is degraded at each cell division. cdc13 and cdc2 co-localize in the cell nucleus. cdc2 is not degraded during mitosis, but in the absence of cdc13 it is not localized in the nucleus. These observations suggest that the cdc13+-encoded cyclin acts to regulate both the catalytic properties and the localization of the protein kinase of which it is a subunit.