Abstract: Using the frequency of sister chromatid exchanges (SCE) as an indicator, we have developed an in vivo system for assessing environmental mutagenic agents and water pollutants which act upon fish. The sister chromatid differential (SCD) and SCE were demonstrated by thionine-UV-Giemsa procedures 5 days after the fish having been injected with 500 μg/g of BrdU (5-bromodeoxyuridine). The frequencies of SCD and SCE in renal cells of grass carp were found to be 8.58±0.22% and 3.05±2.523 per metaphase, respectively. By using Mitomycin C (MMC) as a positive control, the abilities of MNNG and Mipc to induce SCE were analyzed. This system is considered to be a useful tool for water pollution monitoring.