Abstract: Lichens synthesize very special metabolites as differ from other plant species. Usnea longissima, which is usually spread in moist and shady areas, is a lichen species. Usnea longissima has many biological activities. Diffractaic acid is one of the major metabolites of Usnea species. The present study aimed to investigate the anticancer efficacy of a lichen metabolite, diffractaic acid, on experimentally induced in vivo Ehrlich ascites carcinoma. Diffractaic acid was isolated and its chemical structure was confirmed by UV, IR, 1H-NMR, 13C-NMR, 1D and 2D-NMR spectroscopic methods. A total of 50 Balb/C male mice were divided into 5 groups (n:10). About 1x106 carcinoma cells were inoculated intraperitonally to the animals and after 2 days 50-200 mg kg-1 diffractaic acid were given for 13 days. Heamatological parameters were measured from serum samples. The EAC fluid was collected by paracentesis and the cells were counted according to trypan blue dye exclusion method. Stomach, liver, kidney and small and large intestine tissues were examined histopathologically. Diffractaic acid showed the antitumor effect on EAC cells and histopathological and haematological studies showed lower doses of diffractaic acid has protective effect as compared with its higher doses.