In sera of 72 patients with lung cancer, 20 patients with various benign lung diseases and 34 age matched controls circulating immune complexes were determined by column chromatography on Sepharose 6 B and subsequent testing of the eluate for macromolecular IgG as well as by inhibition of radiolabelled C1q binding to sensitized sheep erythrocytes. Whereas in both control and benign lung disease-sera complexes could be detected in less than 5%, sera of lung cancer patients showed macromolecular IgG in 83% and C1q reactive material in 53% at the time of diagnosis. Patients with metastases exhibited a significantly higher percentage of positive reactions than those without metastases (macromolecular IgG 93%/68%, C1q 71%/28%). The size of the complexes increased with the extent of disease. So far, no signficiant changes in circulating immune complexes could be demonstrated id pretherapeutic values were compared with those after X-ray-, chemo- or immunotherapy with one exception, which is an increase of C1q reactive material after radiotherapy.