The suppressing effects of crude extracts of three kinds of tea-green tea (GT) from Japan, Po-lei tea (PT) from China, and Rooibos tea (RT) from South Africa-on the induction of chromosome aberrations in cultured CHO cells and mice were studied. When CHO cells were exposed to each tea extract in the presence of rat liver microsomal enzymes (S9 mix) together with benzo[a]pyrene (B(a)P) or mitomycin C (MMC), a decrease in the frequency of chromosome aberrations was observed. PT and RT, but not GT, also suppressed the induction of chromosome aberrations by MMC in the absence of S9 mix. When cells were treated with tea extract after B(a)P or MMC treatment, RT suppressed the induction of chromosome aberrations in the presence and absence of S9 mix whereas GT and PT showed suppressing effects only in the presence of S9 mix. These data suggest that catechines, well-known antimutagens in tea samples, might account for the inhibitory effect in the case of GT and PT. Since RT contains few catechines, several unknown antimutagenic components could be responsible for its effect. The antimutagenic effects of tea extracts at concentration levels consumed by humans were examined in mice using micronucleus induction with B(a)P or MMC. When mice received oral gavage of 0.2% GT, 0.1% PT, and 0.1% RT at 1.0 ml/mouse 6 h before intraperitoneal injection of MMC, a decrease in the frequency of micronuclei was observed. The induction of micronuclei by B(a)P was suppressed by oral dosage of GT, PT and RT at 1.0 ml/mouse/day for 28 days. This was not due to a delay in the maturation of micronucleated reticulocytes. In conclusion, intake of tea might suppress the mutagenic activity of certain potent mutagens in human beings.