We investigated the role of glycogen synthase kinase-3 (GSK-3), which is inactivated by AKT, for its role in the regulation of apoptosis. Upon IL-3 withdrawal, protein levels of MCL-1 decreased but were sustained by pharmacological inhibition of GSK-3, which prevented cytochrome c release and apoptosis. MCL-1 was phosphorylated by GSK-3 at a conserved GSK-3 phosphorylation site (S159). S159 phosphorylation of MCL-1 was induced by IL-3 withdrawal or PI3K inhibition and prevented by AKT or inhibition of GSK-3, and it led to increased ubiquitinylation and degradation of MCL-1. A phosphorylation-site mutant (MCL-1(S159A)), expressed in IL-3-dependent cells, showed enhanced stability upon IL-3 withdrawal and conferred increased protection from apoptosis compared to wild-type MCL-1. The results demonstrate that the control of MCL-1 stability by GSK-3 is an important mechanism for the regulation of apoptosis by growth factors, PI3K, and AKT.