Green tea, a product of the dried leaves of Camellia sinensis, is the most widely consumed beverage in the world. The polyphenolic compounds from green tea (PGT) possess antiinflammatory properties. We investigated whether PGT can afford protection against autoimmune arthritis and also examined the immunological basis of this effect using the rat adjuvant arthritis (AA) model of human rheumatoid arthritis (RA). AA can be induced in Lewis rats (RT.1(l)) by immunization with heat-killed Mycobacterium tuberculosis H37Ra (Mtb), and arthritic rats raise a T cell response to the mycobacterial heat-shock protein 65 (Bhsp65). Rats consumed green tea (2-12 g/L) in drinking water for 1-3 wk and then were injected with Mtb to induce disease. Thereafter, they were observed regularly and graded for signs of arthritis. Subgroups of these rats were killed at defined time points and their draining lymph node cells were harvested and tested for T cell proliferative and cytokine responses. Furthermore, the sera collected from these rats were tested for anti-Bhsp65 antibodies. Feeding 8 g/L PGT to Lewis rats for 9 d significantly reduced the severity of arthritis compared with the water-fed controls. Interestingly, PGT-fed rats had a lower concentration of the proinflammatory cytokine interleukin (IL)-17 but a greater concentration of the immunoregulatory cytokine IL-10 than controls. PGT feeding also suppressed the anti-Bhsp65 antibody response. Thus, green tea induced changes in arthritis-related immune responses. We suggest further systematic exploration of dietary supplementation with PGT as an adjunct nutritional strategy for the management of RA.