The sera of 80 newly diagnosed lung-cancer patients have been examined for immune complexes and autoantibodies. Control subjects consisted of 20 bronchitic patients and 150 normal blood donors. Immune-complex measurements used 4 established and sensitive techniques (Raji cell assay, fluid and solid-phase C1q assays and conglutinin-binding assay) and a 5th newly devised technique based on the binding of polyethylene-glycol-precipitated immune-complex-rich serum fractions to Staphylococcus aureus. Using the Raji cell assay and the S. aureus binding assay to measure immune complexes, both newly diagnosed lung cancer patients and bronchitic patients had significantly higher prevalences of immune complexes than normal controls, but the two groups of patients did not differ significantly in either prevalence or quantity of immune complexes. When techniques which depend solely upon complement fixation (C1q assays and conglutinin binding) were used, only meagre quantities of immune complexes were found, and in at most 15% of newly diagnosed lung-cancer patients. The presence of autoantibodies in newly diagnosed cancer patients and controls appeared to correlate with the increase in the detectable prevalence of immune complexes.