The ubiquitin-mediated degradation of mitotic cyclins is required for cells to exit from mitosis. Previous work with cell-free systems has revealed four components required for cyclin-ubiquitin ligation and proteolysis: a nonspecific ubiquitin-activating enzyme E1, a soluble fraction containing a ubiquitin carrier protein activity called E2-C, a crude particulate fraction containing a ubiquitin ligase (E3) activity that is activated during M-phase, and a constitutively active 26S proteasome that degrades ubiquitinated proteins. Here, we identify a novel approximately 1500-kDa complex, termed the cyclosome, which contains a cyclin-selective ubiquitin ligase activity, E3-C. E3-C is present but inactive during interphase; it can be activated in vitro by the addition of cdc2, enabling the transfer of ubiquitin from E2-C to cyclin. The kinetics of E3-C activation suggest the existence of one or more intermediates between cdc2 and E3-C. Cyclosome-associated E3-C acts on both cyclin A and B, and requires the presence of wild-type N-terminal destruction box motifs in each cyclin. Ubiquitinated cyclins are then rapidly recognized and degraded by the proteasome. These results identify the cyclosome-associated E3-C as the component of the cyclin destruction machinery whose activity is ultimately regulated by cdc2 and, as such, the element directly responsible for setting mitotic cyclin levels during early embryonic cell cycles.