Tanacetum vulgare L. (Tansy) has been extensively used in folk medicine for treatment of a variety of medical disorders. In the present study, we isolated and purified four acidic polysaccharide fractions (designated T-I to T-IV) from Tansy florets by the sequential use of hot-water extraction, ethanol precipitation, ultra-filtration, anion-exchange, and size-exclusion chromatography. The average M(r) of fractions T-I through T-IV was estimated to be 326, 151, 64 and 9 kDa, respectively, as determined by high performance size-exclusion chromatography analysis. Sugar composition analysis revealed that Tansy polysaccharides consisted primarily of galacturonic acid, galactose, arabinose, and rhamnose. Fractions T-II through T-IV contained an arabinogalactan type II structure, as determined by reaction with Yariv reagent. High M(r) fractions T-I and T-II exhibited potent macrophage/monocyte-activating activity, enhancing production of reactive oxygen species (ROS), nitric oxide (NO), and tumor necrosis factor alpha (TNF-alpha) by J774.A1 murine macrophages, and activating nuclear factor kappaB (NF-kappaB) in THP-1 human monocytes. In addition, Tansy polysaccharides stimulated human neutrophil function by greatly enhancing neutrophil myeloperoxidase (MPO) release. Furthermore, the low M(r) fraction T-IV had potent complement-fixing activity, which may also contribute to the anti-inflammatory and would-healing properties of Tansy extracts. Taken together, our results provide a molecular basis to explain at least part of the beneficial therapeutic effects of Tansy extracts, and support the concept of using Tansy polysaccharides as an immunotherapeutic adjuvant.