Silymarin is a mixture of bioactive flavonoids isolated from Milk Thistle (Silybum marianum). Crude extracts from this plant have been used for centuries as a natural remedy and silymarin is now effectively used in the treatment of inflammatory liver toxicity and disease in humans. In vitro studies show that silymarin can inhibit the production and damage caused by tumor necrosis factor alpha (TNFalpha) and is a potent antioxidant both in vitro and in vivo. Such findings suggest silymarin may impact the immune system but little information exists following in vivo exposure. Therefore, we tested the hypothesis that exposure to silymarin will modulate the inflammatory immune response. Male BABL/c mice (6/group) were treated intraperitoneally once daily for five days with 0, 10, 50 or 250 mg/kg of silymarin. Silymarin exposure did not produce any signs of overt toxicity or any changes in relative organ weights. Flow cytometric examination of splenic lymphocyte populations showed that the absolute number of CD3+ T-lymphocytes was reduced in the 10 and 50 mg/kg groups although significance was evident only in the 10 mg/kg group. Concomitant decreases in CD4+ and CD8+ T-cell populations were observed but only the CD4+ population in mice treated with 10 mg/kg of silymarin was significantly different from control. Functional examination of secondary lymphoid cells revealed that phytohemagglutinin-induced T-lymphocyte proliferation was increased in the lowest dose group only. B-lymphocyte blastogenesis induced by lipopolysaccharide was increased following exposure to 10 and 50 mg/kg of silymarin. Similarly, expression of TNFalpha, inducible nitric oxide synthase, IL-1beta and IL-6 mRNA were increased dose-dependently. The expression of IL-2 and IL-4 were reduced in mice treated with 10 and 50 mg/kg of silymarin although only the 10 mg/kg group was significantly different from control. The results indicate that in vivo parenteral exposure to silymarin results in suppression of T-lymphocyte function at low doses and stimulation of inflammatory processes at higher doses. Further studies investigating the effects of silymarin on the immune system are warranted.