It is postulated that the mobile inflamed joint may be subject to cyclical ischaemic reperfusion injury. Xanthine oxidoreductase is an enzyme thought to contribute to oxidative reperfusion injury, and the detection of this activity in human synovium is described. Three normal and five rheumatoid tissues were assayed with a carbon-14 radioassay detecting the conversion of [14C]xanthine to [14C]uric acid. Rheumatoid synovia contained 0.67-305 microU/g tissue (n = 5), while normal synovia contained 1.2-5.0 microU/g tissue (n = 3).