Application of comet assay (single cell gel electrophoresis assay) to detect DNA damaging potential in the environmental water was examined in this study. Cultured human lymphocyte cell line was served as target organism and contacted in vitro with concentrated leachate samples from a waste landfill. Dose-response relationship was confirmed with positive DNA damaging reagents (B(a)P and 4NQO). Comet assay could detect statistically positive DNA damage in 3-folds concentrated leachate with S9mix metabolic activation. DNA damaging potential in the samples seemed to be caused by ash, which consists most of disposed wastes on the landfill. Differences of detection sensitivity and characteristic between comet assay and conventional microbiological mutagenic tests were also discussed.