Galectin-8, a mammalian beta-galactoside binding lectin, functions as an extracellular matrix protein that forms high-affinity interactions with integrins. Here we demonstrate that soluble galectin-8 inhibits cell cycle progression and induces growth arrest. These effects cannot be attributed to interference with cell adhesion, but can be attributed to a 4-5-fold increase in the cellular content of the cyclin-dependent kinase inhibitor p21, which is already evident following 4 h incubation of H1299 cells with galectin-8. The increase in p21 levels is preceded by 3-5 fold increase in JNK and PKB activities. Accordingly, SP600125, the inhibitor of JNK, and wortmannin, the inhibitor of PI3K, which is the upstream activator of PKB, inhibit the increase in the cellular content of p21. Furthermore, overexpression of a dominant-inhibitory form of SEK1, the upstream kinase regulator of JNK, inhibits both JNK activation and p21 accumulation. When p21 expression is inhibited by cycloheximide, galectin-8 directs the cells towards apoptosis, which involves induction of PARP cleavage. Indeed, galectin-8-induced apoptosis is two-fold higher in HTC (p21-null) cells, when compared to parental HTC cells. Because overexpression of galectin-8 attenuates the rate of DNA synthesis, stable colonies which overexpress and secrete galectin-8 can be generated only in cells overexpressing growth factor receptor, such as the insulin receptor. These results implicate galectin-8 as a modulator of cellular growth through up regulation of p21. This process involves activation of JNK which enhances the synthesis of p21, combined with activation of PKB that inhibits p21 degradation. These effects of the lectin depend upon protein-sugar interactions and are induced when galectin-8 is present as a soluble ligand or when it is overexpressed in cells.