Progression through the G1/S transition of the cell cycle is regulated by cyclin E/cdk2 and cyclin A/cdk2 complexes. We demonstrate that there are two forms of murine cdk2 (cdk2α and β). Cdk2α consist of 298 amino acids, while cdk2β contains a 48-amino-acid insert between Met (196) and Val (197) of cdk2α. Cdk2β results from differential splicing of the primary RNA transcript of the cdk2 gene. Although human cdk2 genomic DNA contained the sequence of the insert for the β form, cdk2β was not detected by either Western blot or RT-PCR in human T-cells or several other human cell lines. Cdk2β expression in murine cells was similar to that of the phosphorylated, catalytically active form of cdk2α. Cdk2α and cdk2β have very similar binding activity to cyclin E and to the cdk inhibitor p27Kip1. The alternatively spliced cdk2β possesses catalytic activityin vivoandin vitro.The differential catalytic activity of these two forms of cdk2 suggests that cdk2α and cdk2β may perform different functions at or near the G1/S transition and early S phase.