Five of twenty-three patients with paraproteinaemia (two IgM, three IgG) have been shown to exhibit marked classical pathway complement activation. The mechanisms of hypocomplementaemia proposed for the five patients are cryoglobulinaemia in one and in vivo immunoglobulin aggregation in the other four. Three further patients had a low C1q and three a low C3 unassociated with any other complement abnormality. No association with any particular IgG subclass or obvious clinical abnormality existed in association with hypocomplementaemia.