PROGRESSION Of the eukaryotic cell cycle is controlled by cyclin-dependent kinases (CDKs)(1). Cdc28, the budding yeast homologue of Cdc2 (Cdk1), is required for both the G1/S and G2/M transitions of the cell cycle(2), The functional specificity of the Cdc28 kinase is determined by its association with G1 or G2 cyclins. Alternation of cell cycle phases is thus mainly due to mechanisms that ensure that one cyclin family succeeds another, Here we show that the G2 cyclins Clb1, Clb2, Clb3 and Clb4 are required for the proteolysis of the G1 cyclins Cln1 and Cln2, providing a mechanism for coupling synthesis of G2 cyclins with the disappearance of G1 cyclins. Our data indicate that this pathway involves the Ubc9 ubiquitin-conjugating enzyme(3). The Cdc34 ubiquitin-conjugating activity(4) may function redundantly with Ubc9, or it may only be involved in Cln1,2 turnover through its role in promoting the degradation of Sic1, a specific inhibitor of Cdc28-Clb complexes(5).