Polygonum tinctorium Lour. (P. tinctorium) is known to have the ability to suppress inflammation. We attempted to isolate the active compounds from P. tinctorium based on their inhibitory effects on the production of interferon-gamma, which is a well-known inflammatory cytokine. We thus isolated indirubin, an isomer of indigo. Indirubin exerted its inhibitory effects not only on interferon-gamma production by human myelomonocytic HBL-38 cells but also on interferon-gamma and interleukin-6 production by murine splenocytes with no influence on the proliferation of either cells. Because of its inhibitory activity on interferon-gamma production, we further investigated the effects of indirubin on 2,4, 6-trinitro-l-chlorobenzene (TNCB)-induced delayed-type hypersensitivity as a representative inflammatory reaction. When injected intraperitoneally, indirubin significantly inhibited the ear swelling of TNCB-elicited mice. The amount of interferon-gamma in the culture supernatants of elicited mouse lymphocytes was inhibited by indirubin treatment. These results suggest that indirubin is a compound with anti-inflammatory effects.