The effects of ascorbic acid, iron and ADP on hyaluronic acid, a compound present in inflamed joints, were investigated in an in vitro system. Ascorbic acid induces degradation of hyaluronic acid which increased in the presence of FeCl3 and which is additionally stimulated by ADP chelated ferric ions. The hyaluronic acid degrading reactions induced by the Fe-III/ADP/ascorbic acid system were inhibited by catalase and formate to various extents whereas the presence of superoxide dismutase did not exert any inhibitory effect. Desferrioxamine, a specific iron chelator, completely inhibited hyaluronic acid depolymerisation by ascorbic acid as well as in combination with FeCl3 or FeCl3/ADP, respectively. We suggest that the ultimate hyaluronic acid degrading species is OH, generated via the Fe-III/ADP catalysed Haber Weiss reaction. There is also an indication for the involvement of perferryl or/and ferryl species in the degradation process.