Cancer chemopreventive effects of organic extracts from 29 species of lichens collected in Iceland were evaluated using a panel of in vitro bioassays whereby extracts were tested for potential to induce quinone reductase (QR) and differentiation of human promyelocytic leukemia (HL-60) cells, inhibit cyclooxygenase-1 (COX-1), phorbol ester-induced ornithine decarboxylase (ODC), aromatase and sulfatase, as well as for antioxidant, estrogenic/anti-estrogenic and antiproliferative activity. In addition, the extracts were tested for cytotoxicity against 12 cancer cell lines. The most significant results were exhibited by extracts from Xanthoria elegans and Alectoria nigricans , which respectively, induced QR activity (concentration to double activity = 4.8 µg/ml) and inhibited phorbol ester-induced ODC activity with mouse 308 cells in culture (IC 50 = 2.6 µg/ml). Moderate inhibition of [ 3 H]thymidine incorporation with HL-60 cells was exhibited by the Peltigera leucophlebia extract. Several extracts prevented estrogen formation from estrogen precursors by inhibiting the enzymatic activities of aromatase ( Sphaerophorus globosus , Cetrariella delisei , Melanelia hepatizon ) and sulfatase ( Cladonia gracilis , Sphaerophorus fragilis , S. globosus ). None of the extracts demonstrated significant cytotoxic effects with selected cell lines.