THE mechanism underlying chronic inflammation is of considerable medical importance both in temperate countries—where diseases such as rheumatoid arthritis are common—and in tropical countries—where various chronic inflammatory conditions cause much disease and disability. It is generally accepted that the macrophage is the central cell in chronic inflammation. We have drawn attention to the parallelism between the capacity of various agents to induce chronic inflammation in vivo and selective lysosomal enzyme secretion from cultures of macrophages1,2. The mechanism by which macrophage enzyme secretion is induced is unknown: we now report a mechanism mediated by cleavage products of the complement component C3.