SummaryExcess anticomplementary activity is common in the sera of patients with bronchiectasis (chronic bronchial suppuration). To identify the active components, Sephadex 6B gel filtration profiles of the sera of fourteen patients with bronchiectasis were obtained, six of which were subjected to detailed analysis for anticomplementary activity and IgG and IgM content. Anticomplementary serum from an individual patient with rheumatoid arthritis was similarly analysed as a positive control and sera from four healthy subjects as negative controls.Although some anticomplementary activity was monitored in association with monomeric IgG, in patients' sera activity was predominantly, and occasionally exclusively, associated with the region between the native IgM and IgG peaks. High molecular weight IgG was detected by immunodiffusion in these fractions of the sera from three patients with severe bronchiectasis, but was undetectable in that of the other three patients, in whom disease was mild. Only one of the four normal sera was anticomplementary, weak activity being distributed in the IgM, IgG and intermediate fractions.The data are interpreted to suggest the presence of immune complexes of IgG in the sera of patients with severe bronchiectasis. Failure to consistently detect IgG in column fractions exhibiting anticomplementary activity is probably a reflection of the superior sensitivity of the anticomplementary technique. Gel filtration properties indicate that the immune complexes are small, containing less than five molecules of IgG. Their possible role in the pathogenesis of bronchiectasis, or as a secondary response to chronic infection and pulmonary tissue damage, is discussed.