Terrestrial plants have long been considered to be a rich source of biologically active secondary metabolites. Previously many plants belonging to the mint family (Labiatae) showed antiviral activity against a number of viruses, including HIV-1. Tannins found in mint plant extracts and as secondary metabolites in food, including tea have been shown to have moderate anti-HIV activity. Earlier reports on Hyssop officinalis revealed that the crude methanolic and aqueous extracts contained strong anti-HIV l activity and the anti-HIV-1 activity was due to substances other than tannins. These reports prompted us to isolate and investigate the active components of the extract of Hyssop officinalis. Our in vitro study demonstrates strong anti-HIV-1 activity of a polysaccharide (MAR-10) isolated from the aqueous extract of Hyssop officinalis without any untoward side effects on lymphocyte subsets and their functions.