Research studies in drug discovery are progressing with high speed due to the introduction of combinatorial chemistry and robotics. Therefore, genotoxicity screening assays which can be conducted with a small amount of compound, in a short time, and which could predict the Ames test results are required in the early stage of research. We therefore established potentially useful assays that could be run in microwell plates. The fluctuation Ames test (FAT) using 384-well plates and Salmonella typhimurium TA98 and TA100, and the SOS/ umu assay (SOS) using 96-well plates with S. typhimurium TA1535/pSK1002 and NM2009 as based on FAT reports from Gatehouse et al. (1979) and Gee et al. (1998) and on SOS reports from Reifferscheid et al. (1991) were developed. Seventeen Ames positive mutagens were tested with both assays in the presence and absence of S9 mix. The percentage of correctly predicted compounds was 88 % (15/17), 88 % (15/17), and 94 % (16/17) in the FAT, SOS and FAT + SOS, respectively. The amount of compound required for the Ames test (pre-incubation method with TA98 and TA100), FAT, and SOS is 100 mg, 5 mg, and 1 mg, respectively. Practical numbers of compounds tested and cost of testing one compound is 12, 20, and 54 compounds per week, and ¥20000, ¥6400, and ¥2400, respectively. The FAT and SOS are also able to be automated, and are therefore considered useful as high throughput genotoxicity screening assays.