Grass Carp(Ctenopharyngodon idellus) was chosen as a experimental fish in the present and the results which were obtained from it are as following: First, a diploid fish cell line, GCTF-2, from Grass Carp tail fin was established. The line was cultivated in TC-199 medium with 15% fetal calf serum containing a moderate amount of antibiotics. The cells can increase five times in five days and the cell morphology is a complex of epithelial-like cell and fibroblasticlike cell. The highest index of mitotic division of GCTF-2 is 3.004% in 48 hours after plating. The optimal grown temperature is 28 ℃, though the cells will grow at temperature from 20-35 ℃. A interesting discovery we did was that the cells which had been kept in the incubator at 20 ℃ for two months without any care could grow normally after plating. At 90 generation, the cells which had a normal number of chromosomes of Grass Carp (2n = 48) accounted for 87.6%, so the line was determined as a diploid fish cell line. By the end of January 1985, the line had been maintained in proliferation for over 17 months through 100 generations. Using the techniques of BrdU-labling and sister chromatid differential staining, we have estimated the cell cycle time of mitotic division of CGTF-2 between 21-33 hours. GCTF-2 was observed to be susceptible infection with Grass Carp Reovirus(GCRV). Second, utilizing the technique of sister chromatid differential staining and considering the sister chromatid exchange frequency as a indicator, we have investigated the spontaneous SCE frequency of GCTF-2 cells and the baseline SCE frequency of Grass Carp renal cells (in vivo) and the possible mutagen activities of 4 chemicals (mitomycin C, MMC; N-methyl-N'-nitro-N-nitrosoguanidine, MNNG; MIPC; and Bayleton) in the cultures of GCTF-2 diploid cells and Grass Carp renal cells. The experimental results indicated that the spontaneous SCE frequencies of GCTF-2 and the baseline SCE frequencies of Grass Carp renal cell are low and there is not significat differentiation between them; All 4 chemicals can induce significant increase of SCE frequency of GCTF-2 cells at the concentrations we used. MMC and MNNG can induce remarkable increase of SCE frequencies of Grass Carp renal cells, but MIPC can not. In addition, MMC and Bayleton have a activity of cell-cycle deley on GCTF-2 cells, and MMC has no effect on frequency of SCD in renal cells of Grass Carp in vivo at the concentrations we used. From above, using the indicator of SCE, we can firstly establish a susceptible test system measuring chemical mutagen and carcinogenic substances in vitro and in vivo in fish, and it may be considered that MIPC and Bayleton are two relatively safe pesticides.