To examine the role of circulating immune complexes (CIC) in infective endocarditis, we studied 64 patients with infective endocarditis for the presence of CIC by the polyethylene-glycol (PEG)-precipitation test and the Clq binding test. This study was repeated during the course of the disease in 23 patients. CIC were found in 84 per cent of patients (66 per cent with acute infective endocarditis, 89 per cent with subacute infective endocarditis) during the active phase of the disease. Higer PEG precipitates were associated with typical cutaneous signs, cryoglobulins and nonstreptococcic culture-positive infective endocarditis. Under appropriate antibiotic treatment, the PEG precipitate levels of 17 patients fell within 1 month to the normal range, with a concomitant drop in cryoglobulinemia and rheumatoid factor. Conversely, uncontrolled sepsis always (six of six) yielded a rising level of CIC. These findings support the hypothesis that CIC may be important in the pathogenesis of peripheral lesions in infective endocarditis.