The effect of glycogen synthase kinase 3beta (GSK3beta) has been repeatedly implicated in cell proliferation, but studies on the effect of GSK3beta in different cell lines with different stimuli have drawn different conclusions. To investigate the direct effect of GSK3beta on cell growth in human lung adenocarcinoma cell line A549, we changed its activity by transient transfection with two kinds of GSK3beta mutant plasmids, constitutively active form S9A-GSK3beta and dominant negative form KM-GSK3beta. Twenty-four hours later, cell counting, flow cytometry and Western blot detection were made respectively. The results showed that enhancing GSK3beta activity caused a decrease in cell number, as well as a higher percentage of cells at G(1) phase. Further, the expression of cyclin D1 was down-regulated by GSK3beta. Taken together, our observations suggest that GSK3beta may induce G(1) cell cycle arrest in a cyclin D1-dependent fashion and therefore possibly plays a growth-inhibitory role in A549 cells.