Of several lichen metabolites isolated from Parmelia nepalensis and Parmelia tinctorum, the didepsides atranorin (4) and diffractaic acid (5), as well as (+)-protolichesterinic acid (7), inhibited LTB4 biosynthesis in polymorphonuclear leukocytes. Ethyl hematommate (3) and (+)-usnic acid (1) were only weak inhibitors, while methyl beta-orcinolcarboxylate (2) and gyrophoric acid (6) were inactive at concentrations up to 60 microM. Redox properties of the compounds were evaluated in terms of inhibition of nonenzymatic lipid peroxidation in model membranes, reactivity against the stable free radical 2,2-diphenyl-1-picrylhydrazyl, and deoxyribose degradation as a measure of hydroxyl-radical generation. The results revealed that lichen metabolites neither acted as antioxidants against the peroxidation process in model membranes nor did they scavenge or produce free radicals, suggesting that the inhibitory effects on LTB4 biosynthesis was due to specific enzyme interaction rather than a nonspecific redox mechanism.