Poncirin which is one of the flavanone rhamnoglucosides showed anti-inflammatory activity as the major component of fruit of Poncirus trifoliata. Poncirin did not show antiinflammatory effect when it was intraperitoneally administered, but it was very effective when orally administered. Poncirin was not metabolized by blood and liver enzymes but by intestinal bacteria of human and rats. Among the human intestinal bacteria, Streptococcus Y-25 converted poncirin to ponciretin through the poncirenin and Staphylococcus Y-88 converted to ponciretin directly.