Many tumorigenic processes affect cell-cycle progression by their effects on the levels of the cyclin-dependent kinase inhibitor p27^Kip1[1,2]. The phosphorylation- and ubiquitination-dependent proteolysis of p27 is implicated in control of the G1–S transition in the cell cycle [3–6]. To determine the factors that control p27 stability, we established a cell-free extract assay that recapitulates the degradation of p27. Phosphorylation of p27 at Thr187 was essential for its degradation. Degradation was also dependent on SCF^Skp2, a protein complex implicated in targeting phosphorylated proteins for ubiquitination [7–10]. Immunodepletion of components of the complex – Cul-1, Skp1, or Skp2 – from the extract abolished p27 degradation, while addition of purified SCF^Skp2 to Skp2- depleted extract restored the capacity to degrade p27. A specific association was observed between Skp2 and a p27 carboxy-terminal peptide containing phosphorylated Thr187, but not between Skp2 and the non-phosphorylated peptide. Skp2-dependent associations between Skp1 or Cul-1 and the p27 phosphopeptide were also detected. Isolated SCF^Skp2 contained an E3 ubiquitin ligase activity towards p27. Our data thus suggest that SCF^Skp2 specifically targets p27 for degradation during cell-cycle progression.