The effects of exogenous p16(ink4a) gene on biological behaviors of human lung cancer cell line with homozygous deletion of p16(ink4a) gene were investigated. Exogenous p16(ink4a) gene was transfected by lipofectin into human lung cell line A549, in which p16(ink4a) gene was homozygously deleted. The expression of p16(ink4a) mRNA and protein was detected by RT-PCR and immunocyto-chemistry, respectively. The changes in the behaviors of the transfected cell lines in vitro and in vivo were observed. In the transfected cell line A549, the exogenous p16(ink4a) gene could be stably expressed. The growth of A549 cells transfected with p16(ink4a) gene was obviously slowed down. Flow cytometry revealed that transfection of the exogenous p16(ink4a) gene resulted in A549 cell lines arrest in G1 phase of cell cycle. The tumorigenicity of these transfected cells in nude mice could be inhibited, and the tumor growth of nude mice was significantly suppressed. It was concluded that exogenous p16(ink4a) gene may be stably expressed in human lung cancer cell line A549. The expression of the introduced p16(ink4a) could block lung cancer cells to entry into S phase of cell cycle and inhibit tumor malignant growth both in vitro and in vivo.