An in vitro sister-chromatid exchange (SCE) assay using rat erythroblastic leukemia cells was conducted with four major trihalomethanes (THMs): chloroform, CHCl3; dichlorobromomethane, CHCl2Br, dibromochloromethane, CHClBr2; bromoform, CHBr3. In the absence of S9 mix, CHBr3, CHClBr2 and CHCl2Br significantly induced SCEs in a clear dose-dependent manner, while CHCl3 did not significantly induce SCEs. On the other hand, the incidence of CHCl3-induced SCEs significantly increased, although the incidence of CHBr3-induced SCEs decreased by the addition of S9 mix. However, there was no difference between the incidence of SCEs induced by CHBr3, CHClBr2 or CHCl2Br in the absence of S9 mix and that in the presence of S9 mix. The addition of crude catechin to the SCE assay system suppressed the ability of CHCl3 or CHBr3 to induce SCEs but had no suppressive effect on the other THM-induced SCEs. The suppression of SCEs induced by CHCl3 or CHBr3 depended on the crude catechin dose.