Kinases of the mammalian cdc2 family including cdk2 (cyclin-dependent kinase 2) are thought to be involved in both the G2/M transition and DNA replication. To investigate the role of cdc2 kinase and cdk2 in cell cycle progression, murine tsFT210 cells bearing a temperature-sensitive cdc2 mutation were used. These kinases were purified by column chromatography, using a peptide with the consensus phosphorylation site of cdc2 kinase as the substrate. In this mutant, cdc2 kinase activity was temperature sensitive and cdk2 activity was not. At the restrictive temperature, the mutant was only arrested in the G2 phase and not in the G1-S phase, suggesting that cdk2 did not compensate for cdc2 kinase at the G2/M transition but did function at the G1-S phase. This suggestion was supported by the finding that transfection of cdk2 cDNA did not improve the growth of the mutant cell line at the restrictive temperature, although transfection of cdc2 cDNA did.