Some nonspecific components (IgG, C3c, C4,) of circulating immune complexes (IC) precipitable by 3.5% PEG were assayed by laser nephelometry in the sera of 71 patients with solid tumours, and of 39 patients with autoimmune diseases. PEG-IgG, PEG-C3c, and PEG-C4 were higher in cancer and in autoimmune diseases than in controls. In cancer patients, PEG-C4 levels and PEG-C4 positivity rate correlated well with tumour burden. PEG-C3c and PEG-IgG were present at higher levels and PEG-C4 was present at lower concentration in cancer than in autoimmune diseases. The results of the present study, performed on a large population of patients with cancer and autoimmune diseases, indicate a different composition of circulating IC in these diseases. Particularly, these data suggest a preferential involvement of the alternative pathway of complement activation in cancer, as already described by other authors. The aggregation of C3c to IC, mediated by the alternative pathway, seems to be the "key" event in the process of IC solubilization. In cancer patients' sera, the presence of "solubilized" IC could partly explain their peculiar biological behaviour as well as the disagreement observed among the different assay methods based upon the binding with complement factors.