The micronucleus test using peripheral blood reticulocytes (RETs) was evaluated in ICR mice treated with N-methyI-N-nitrosourea (MNU) and benzo(a)pyrene [B(a)P] as model clastogens. The frequency of micronucleated reticulocytes (MNRETs) in both positive compounds was similar to other results which were reported previously. On the other hand, an anticlastogenic effect of the natural antioxidant, $eta$-carotene and one of taroholds, galangin as model anticlastogens were investigated using simultaneous treatment. Mice were treated with a model clastogen alone, or with a model clastogen and a model anficlastogen simultaneously. Both $eta$-carotene and galangin showed anticlastogenic effects against MNU- or B(a)P-induced micronuclei in mice. However, galangin has stronger activity than $eta$-carotene. Results from our experiment suggest that the in vivo supravital staining micronucleus test using peripheral blood is useful in the evaluation of clastogenic and anticlastogenic effects.