Lobaric acid is a depsidone metabolite that has been isolated from Stereocaulon lichen species with antioxidant, antiproliferative, antiviral, and enzyme inhibitory activites. It scavenges superoxide radicals in a cell-free assay (IC50 = 97.9 μmol) and inhibits proliferation in a panel of leukemia, colorectal, gastric, breast, ovarian, prostate, pancreatic, and lung cancer cell lines (EC50s = 15.2-63.9 μg/ml). Lobaric acid inhibits protein tyrosine phosphatase 1B (PTP1B; IC50 = 0.87 μM for the human recombinant enzyme) and production of 12(S)-HETE (Item No. 34570) by 12(S)-lipoxygenase (IC50 = 28.5 μM). In vivo, lobaric acid (250 μM) decreases lesion number, but not lesion diameter, in tobacco leaves infected with tobacco mosaic virus (TMV).