AIM: To investigate the effect of arsenic trioxide on human gastric cancer cell line MKN45 with respect to both cytotoxicity and induction of apoptosis in vitro. METHODS: MKN45 cells were treated with arsenic trioxide(As2O3) at the concentration of 1, 5, and 10 μmol/L,respectively, for three successive days. Cell growth and proliferation were observed by cell counting and trypanblue exclusion. Cytotoxicity of As2O3 was determined by MTT assay. Morphologic changes were studied with light microscopy. Flow cytometry was used to assay cell DNA distribution and apoptotic cells were confirmed with terminal deoxynucleotidyl transferase-mediated dUTP nickend labeling (TUNEL) and DNA electrophoresis.RESULTS: The growth of MKN45 cells was significantlyinhibited by As2O3 which was confirmed by colony-forming assay. After 7 d of culture with various concentrations of As2O3, colony-forming capacity of MKN45 cells decreased with As2O3 increment in comparison with that of control group. The inhibitory rate of colony-formation was 38.5%, 99.1%, and 99.5% when the concentration of As2O3 was1, 5, and 10 iμmol/L in culture medium, respectively. The cell number of a single colony in drug treatment groups was less than that of control group. The cell-killing rate of As2O3 to MKN45 cells was both dose- and timedependent with an IC50 of (11.05±0.25) μmol/L. After incubation in 10 μmol/L As2O3 for 24 h, the cell-killing rate was 27.1%, and it was close to 50% after 48 h. The results showed that As2O3 induced time- and dosedependent apoptosis in MKN45 cells, blocked at G2/M phase. The apoptotic peak (sub-G1 phase) appeared and cell apoptotic rate in MKN45 cells was 18.3-32.5% aftertreatment by 10 μmol/L As2O3 for 48 h. The percentage of G2/M cell of the experimental groups was 2.0-5.0 times than that of the control group. Gel electrophoresis of DNA from cells treated with each concentration of As2O3 for 48 h revealed a ladder patt, indicating preferential DNA degradation at the intucleosomal, linker DNA sections. TUNEL also demonstrated strand breaks in DNA of MKN45 cells treated with As2O3, while control cellsshowed negative labeling.CONCLUSION: As2O3 can induce apoptosis of human gastric carcinoma cells MKN45, which is the basis of its effectiveness. It shows great potential in the treatment of gastric carcinoma.