AIM:To investigate the photocatalytic killing effect ofphotoexcited TiO_2 nanoparticles on human colon carcinomacell line(Ls-174-t)and to study the mechanism underlyingthe action of photoexcited TiO_2 nanoparticles on malignantcells.METHODS:Ls-174-t human colon carcinoma cells werecultured in RPMI 1640 medium supplemented with 199 mL/Lcalf serum in a humidified incubator with an atmosphere of50 mL/L CO_2 at 37℃.Viable cells in the samples weremeasured by using the MTT method.A GGZ-300 W highpressure Hg lamp with a maximum ultraviolet-A(UVA,320-400 nm)irradiation peak at 365 nm was used as light sourcein the photocatalytic killing test.RESULTS:The photocatalytic killing of Ls-174-t cells wascarried out in vitrowith TiO_2 nanoparticles.The killing effectwas weak by using UVA irradiation without TiO_2 nanoparticles.In our studies,the photocatalytic killing effect was correlatedwith the concentration of TiO_2 and illumination time.OnceTiO_2 was added,Ls-174-t cells were killed at a much higherrate.In the presence of 1 000 μg/mL TiO_2,44% of cellswere killed after 10 min of UVA irradiation,and 88% of cellswere killed after 30 min of UVA irradiation.CONCLUSION:When the concentration of TiO_2 is below200 μg/mL,the photocatalytic killing effect on human coloncarcinoma cells is almost the same as that of UVA irradiationalone.When the concentration of TiO_2 is above 200 μg/mL,the remarkable killing effect of photoexcited TiO_2 nanoparticlescan be found.Zhang AP,Sun YP.Photocatalytic killing effect of TiO_2nanoparticles on Ls-174-t human colon carcinoma cells.WorldJ Gastroenterol 2004;10(21):3191-3193http://www.wjgnet.com/1007-9327/10/3191.asp AIM: To investigate the photocatalytic killing effect of photoexcited TiO 2 nanoparticles on human colon carcinoma cell line (Ls-174-t) and to study the mechanism underlying photoecited TiO 2 nanoparticles on malignant cells. METHODS: Ls-174-t human colon carcinoma cells were culture in RPMI 1640 medium supplemented with 199 mL / L calf serum in a humidified incubator with an atmosphere of 50 mL / L CO 2 at 37 ° C. Viable cells in the samples weremeasured by the MTT method. A GGZ-300 W highpressure Hg lamp with a maximum ultraviolet -A (UVA, 320-400 nm) irradiation peak at 365 nm was used as light sourcein the photocatalytic killing test .RESULTS: The photocatalytic killing of Ls-174-t cells wascarried out in vitrowith TiO_2 nanoparticles.The killing effectwas weak by using UVA irradiation without TiO 2 nanoparticles. In our studies, the photocatalytic killing effect was correlatedwith the concentration of TiO 2 and illumination time. OnceTiO 2 was added, Ls-174-t cells were killed at a much higher rate. The presence of 1 000 μg / mL TiO 2, 44% of the cells were killed after 10 min of UVA irradiation, and 88% of the cells were killed after 30 min of UVA irradiation. CONCLUSION: When the concentration of TiO 2 is below 200 μg / mL , the photocatalytic killing effect on human coloncarcinoma cells is almost the same as that of UVA irradiationalone. At the concentration of TiO 2 above 200 μg / mL, the remarkable killing effect of photoexcited TiO 2 nanoparticles was found. Zhang AP, Sun YP. Photocatalytic killing effect of TiO 2 nanoparticles on Ls-174-t human colon carcinoma cells. World J Gastroenterol 2004; 10 (21): 3191-3193 http: //www.wjgnet.com/1007-9327/10/3191.asp