Cyclin-dependent kinases (Cdks) control the eukaryotic cell cycle by phosphorylating serine and threonine residues in key regulatory proteins, but some Cdk family members may exert kinase-independent functions that cannot easily be assessed using gene knockout approaches. While Cdk2-deficient mice display near-normal mitotic cell proliferation due to the compensatory activities of Cdk1 and Cdk4, they are unable to undergo meiotic generation of gametes and are consequently sterile. To investigate whether Cdk2 regulates meiosis via protein phosphorylation or by alternative kinase-independent mechanisms, we generated two different knockin mouse strains in which Cdk2 point mutations ablated enzyme activity without altering protein expression levels. Mice homozygous for the mutations Cdk2D145N/D145N or Cdk2T160A/T160A expressed only 'kinasedead' variants of Cdk2 under the control of the endogenous promoter, and despite exhibiting normal expression of cell cycle regulatory proteins and complexes, both mutations rendered mice sterile. Mouse cells that expressed only 'kinase-dead' variants of Cdk2 displayed normal mitotic cell cycle progression and proliferation both in vitro and in vivo, indicating that loss of Cdk2 kinase activity exerted little effect on this mode of cell division. In contrast, the reproductive organs of Cdk2 mutant mice exhibited abnormal morphology and impaired function associated with defective meiotic cell division and inability to produce gametes. Cdk2 mutant animals were therefore comparable to gene knockout mice, which completely lack the Cdk2 protein. Together, our data indicate that the essential meiotic functions of Cdk2 depend on its kinase activity, without which the generation of haploid cells is disrupted, resulting in sterility of otherwise healthy animals.