In this study, the mutagenic and anti-mutagenic effects of methanol extract of three lichen species (Cetraria aculeata, Cladonia chlorophaea and Cetrelia olivetorum) were investigated by using E. coli-WP2, Ames-Salmonella (TA1535 and TA1537) and sister chromatid exchange (SCE) test systems. The results obtained from bacterial test systems demonstrated that methanol extracts of three lichen species have strong anti-mutagenic potencies on TA1535, TA1537 strains and to a lesser extent on E. coli-WP2 strain. The anti-oxidant level of human lymphocytes cells was determined in order to clarify the mechanism underlying the anti-mutagenic effects of these lichen species. Co-treatments of 5, 10 and 20 mu g/mL concentrations of these three lichen species with AFB decreased the frequencies of SCE and the level of MDA and increased the amount of SOD, GSH and GPx which decreased by aflatoxin. The findings of this work have clearly demonstrated that Cetraria aculeata, Cladonia chlorophaea and Cetrelia olivetorum have significant anti-mutagenic effects which are thought to be partly due to the anti-oxidant activities and the interaction capability of lichen extracts with mutagen agents (Sodium azide, acridin, N-methyl-N'-nitro-N-nitrosoguanidine and aflatoxin B-1).