Cyclin-dependent kinases (Cdks) are key regulators of cell cycle progression and are therefore promising targets for cancer therapy. Cdk1/2 Inhibitor III is a cell-permeable inhibitor of Cdk1/cyclin B and Cdk2/cyclin A (IC50s = 0.6 and 0.5 nM, respectively). It less potently inhibits CDC2-like kinases 1 and 3, VEGFR2, and GSK-3β (IC50s = 8.9, 29, 32, and 140 nM, respectively) and is without effect against a panel of other kinases. Cdk1/2 Inhibitor III blocks the growth of several cancer cell lines (IC50 values range from 20 to 92 nM).