A search was made for circulating immune complexes in 42 patients with biopsy-proven lung cancer; sex- and age-matched healthy volunteers were used as normal controls. Two different mthods were employed: Clq--binding assay and the conglutinin-binding test. Patients and controls were also examined for delayed hypersensitivity by a skin testing using a set of recall antigens. A significant difference was found in the incidence of immune complexes and the response to skin tests between neoplastic subjects and normal controls. However, the incidence of immune complexes was not related to delayed hypersensitivity or the other clinical features investigated.