A microassay was developed for the estimation of macrophage (M phi)-activation by released activated oxygen. From peripheral blood or synovia M phi are isolated. O-2 which is released by activated M phi is photometrically detected by cytochrome c reduction. Differences in O-2-production do not exist between monocytes of rheumatoid arthritis patients and controls. In contrast synovial M phi from rheumatoid arthritis patients show increased levels of O-2-production.