Garlic and garlic products are known to induce anti-inflammatory effects, but much of the research to date has focused on the inhibitory effect of garlic on the activity of mononuclear cells/macrophages. The effect of garlic on the balance of the two mutually inhibitory T helper cell subtypes, Th1 and Th2 cells, has hitherto received little attention. We thus studied the effect of supplementation with garlic oil on the activity of Th1 and Th2 cells. Rats were administered by gavage with garlic oil (10 - 200 mg/kg) or corn oil every other day for 2 weeks. Cervical lymph nodes were collected to assay the lymphocyte proliferation rate and the production of Th1 interleukin 2 (IL-2) and interferon gamma (IFN-gamma) and the Th2 cytokines IL-4 and IL-10 upon stimulation with concanavalin A. Garlic oil enhanced the lymphocyte proliferation rate accompanied by an elevated production of all four cytokines when given at a dose of 100 mg/kg. At 200 mg/kg, the production of IL-4 and IL-10 was further enhanced but IFN-gamma production was suppressed. The ratio of IFN-gamma to IL-4 was enhanced by 50 mg/kg garlic oil but suppressed by 200 mg/kg garlic oil. In conclusion, supplemental garlic oil has a dual effect on Th1-Th2 cell balance: an enhanced T cell response towards the Th1 type at low doses and towards the Th2 type at high doses.