Lipoxygenases (LOXs) have been implicated in carcinogenesis in various cancer types. In the current study, three structurally different lichen metabolites, protolichesterinic acid (1), lobaric acid (2) and baeomycesic acid (3) were tested for anti-proliferative effects against 12 different human cancer cell lines. All compounds have known in vitro 5-LOX inhibitory activity, and 1 and 2 also inhibit 12-LOX. The activity of the lichen metabolites was compared to that of a specific 5-LOX inhibitor, zileuton (4). The following cancer cell lines were tested: Capan-1, Capan-2 and PANC-1 (all from pancreas), T47-D (breast), PC-3 (prostate), NCI-H1417 (small cell lung), NIH:OVCAR-3 (ovary), AGS (stomach), WiDr (colorectal), HL-60, K-562 and JURKAT (acute promyelocytic, erythro- and T-cell leukemia, respectively). Compound 1 showed the greatest inhibitory effect against all cell lines, with EC50 ranging from 2.4 - 18.1 μg mL-1 (7.4 - 55.8 μM), followed by 2, with EC50 of 15.2 - 65.5 μg mL-1 (33.2 - 143.6 μM). The effects of 3 and 4 were of similar orders of magnitude, with EC50 of 28.7 - >80 μg mL-1 (76.8 - > 213.9 μM) and 12.9 - > 80 μg mL-1 (50.4 - > 313.7 μM). The dual 5- and 12-LOX inhibitors 1 and to some extent 2 thus exert significant anti-proliferative effects against a variety of human cancer cell lines, while the selective 5-LOX inhibitors 3 and 4 are considerably less active.