Hyaluronan and its associated water of hydration are the basis of the swelling and edema of acute inflammation. Mystixins are small, synthetic peptides that suppress the acute inflammatory response. Mystixin-7, a prototype of these peptides, has the structure p-anisoyl-Arg-Lys-Leu-Leu-D-Thi-Ile-D-Leu-NH(2). As shown previously by this laboratory, the mystixin-7 peptide inhibits edema formation in vivo following intravenous administration at doses of less than 1.0 mg/kg. Mechanisms by which this peptide might suppress edema were examined here in vitro using cultured cells. Normal human dermal fibroblasts normally secrete large quantities of hyaluronan in response to inflammatory stimuli. Mystixin-7 reduced hyaluronan deposition by up to 80% in such cultures. Stimulation of hyaluronidase activity was observed. Mystixins represent a novel class of anti-inflammatory peptides that suppress the edema associated with inflammation. We propose that stimulation of hyaluronidase activity, with a decrease in net hyaluronan deposition and its associated water of hydration, is among the mechanisms of the anti-inflammatory effect of mystixin peptides.