The passage of mammalian cells through the restriction point into the S phase of the cell cycle is regulated by the activities of Cdk4 and Cdk6 complexed with the D-type cyclins and by cyclin E/Cdk2 (,,). The activities of these holoenzymes are constrained by CDK inhibitory proteins^,. The importance of the restriction point is illustrated by its deregulation in many tumour cells^, and upon infection with DNA tumour viruses. Here we describe the properties of cyclins encoded by two herpesviruses, herpesvirus saimiri (HVS) which can transform blood lymphocytes and induce malignancies of lymphoid origin in New World primates^,, and human herpesvirus 8 (HHV8) implicated as a causative agent of Kaposi's sarcoma and body cavity lymphomas^,. Both viral cyclins form active kinase complexes with Cdk6 that are resistant to inhibition by the CDK inhibitors p16^Ink4a, p21^Cip1and p27^Kip1. Furthermore, ectopic expression of a viral cyclin prevents G1 arrest imposed by each inhibitor and stimulates cell-cycle progression in quiescent fibroblasts. These results suggest a new mechanism for deregulation of the cell cycle and indicate that the viral cyclins may contribute to the oncogenic nature of these viruses.