To find the optimal dose range of chemicals for the in vitro chromosomal aberration test using Chinese hamster cells, the cell growth inhibition test using 'Monocellater' is most commonly used in Japan. Many other tests of cytotoxicity, however, could be used, and some of them actually are. We evaluated six different cytotoxicity tests: (1) cell growth inhibition measured by Monocellater (Olympus Opt. Co., Ltd.), (2) cell growth inhibition by crystal violet staining using 96-well plates, (3) viable cell count by trypan blue staining, (4) lactate dehydrogenase (LDH) activity measurement in culture medium, (5) plating efficiency (direct and re-plating methods), and (6) mitotic index evaluation of slides analyzed for chromosomal aberrations. Chinese hamster lung fibroblast cells (CHL/IU) were treated with four model chemicals (MNNG, KBrO 3 , H 2 O 2 , and one coded detergent) and analyzed by optimized protocols for each assay. All assays showed dose-dependent cytotoxicity, more or less, to all model chemicals, except for the LDH assay, which responded only to the detergent. In general, plating efficiency was too sensitive to use for this purpose. The other four cytotoxic endpoints appear to be acceptable as preliminary dose-finding tests in the in vitro chromosomal aberration test.