Orally administered L-isoleucine, DL-isoleucine and L-leucine exhibited anti-inflammatory activity in many test models of inflammation except formaldehyde-induced inflammation. L-beta-phenylalanine inhibited carrageenan-induced oedema only. L-isoleucine also exhibited prolonged analgesic effect while DL-isoleucine had a short lasting effect. The amino-acids produced no gastric ulceration or overt acute toxicity in doses which effectively suppress inflammation. Anti-inflammatory activity seems to be related with interference with the action and/or synthesis of prostaglandins and deserves further intensive study.