Heme oxygenase-1 (HO-1) which degrades Heme to free iron, biliverdin and carbon monoxide (CO) plays an important role in inflammation. There are, however, conflicting data concerning the role of HO-1 in rheumatoid arthritis (RA) and the therapeutic potential of individual heme degradation products remains to be determined. We therefore investigated the effect of CO and biliverdin upon therapeutic administration in the murine collagen induced arthritis (CIA) model of RA.