A number of anti-inflammatory and other drugs used in the treatment of rheumatoid arthritis have been screened for their ability to cause oxidative damage to lipids and proteins in vitro. Although many drugs exhibited an antioxidant profile, a few drugs tested were pro-oxidant, increasing peroxidation of arachidonic acid by mixtures of haem proteins and H 2 O 2 . This system may be an appropriate model to use in the inflammatory situation, since microbleeding to release haemoglobin occurs in the inflamed rheumatoid joint, where H 2 O 2 is produced by invading neutrophils. The damaging effects of the pro-oxidant drugs phenylbutazone, meclofenamic acid and flufenamic acid were investigated in some detail using this system. Arachidonic acid peroxidation was accentuated in a dose-dependent manner and in the presence of haem proteins and H 2 O 2 , phenylbutazone also causes inactivation of α 1 -antiproteinase, a major serine proteinase inhibitor in biological fluids. The above drugs may interact with ferryl haemoglobin, produced by the reaction of H 2 O 2 with haemoglobin, to generate drug-derived radicals causing oxidative damage in these systems. If such reactions occur in vivo, they could contribute to the side-effects induced by these drugs on administration to certain rheumatoid arthritis patients.