This study of circulating immune complexes was carried out using a C1q radio-labelled complement fixation test in 162 patients suffering from broncho-pulmonary disease. There were a similar number of positive results in primary cancer (44%) or secondary (53%) in tuberculosis (37%) or other pathologies (64%). With primary causes neither the histological type nor the stage of extension were associated with any particular frequency of positive tests. The diagnostic interest of research into circulating immune complexes thus appears limited. This study shows however, the activation of humoral systems of immunity during the course of broncho-pulmonary diseases, and raises the prospect of qualitative studies into the nature of the antigen implicated and their role in the mechanism of defence of the organism.