– ZK-Thiazolidinone, also known as TAL, is an ATP-competitive inhibitor of PLK1. In vitro, ZK-Thiazolidinone counteracts the role of Plk1 in previously established functions, notably, sister chromatid separation, centrosome maturation, and bipolar spindle assembly. In vivo, ZK-Thiazolidinone selectively inhibits PLK1 and causes a prometaphase-like mitotic (G2–M) arrest. ZK-Thiazolidinone inhibits human PLK1 (IC50, 19± 12 nM) and various human and mouse tumor cell lines (IC50, 0.2–1.3 μM). ZK-Thiazolidinone impairs centrosome maturation.