Mercury has been used in many domains of human activities for many years, although in any form mercury is reported to be toxic. On the other hand, lichens have been used in the treatment of several diseases such as tuberculosis, hemorrhoids, ulcer, dysentery and cancer. Animal investigations on some common lichen species have demonstrated their antioxidant and antimutagenic activity. However, there is very scarce data on the medical or biologic effects of specific lichen species. Therefore, in the present study, we assessed the cyotogenetic effects of mercuric chloride (HgCl(2)) and the role of aqueous Dermatocarpon intestiniforme lichen extracts in mercury-treated human blood cultures (n = 3). The sister chromatid exchange (SCE) and micronucleus (MN) assays were performed to assess DNA damages in lymphocytes. Our results clearly revealed that the SCE and MN rates induced by HgCl(2) were alleviated by the presence of D. intestiniforme. In conclusion, the results of the present study revealed for the first time that the lichen D. intestiniforme provided increased resistance of DNA against HgCl(2)-induced genetic damage on human lymphocytes.