The serum levels of circulating immune complexes (IC) were evaluated by a new 'direct' laser nephelometric assay, concomitantly with four acute phase proteins (APP), in 167 patients with various neoplasms at different stages. The present study confirms the presence of IC in the sera of cancer patients and, in the breast cancer group, there is a significant correlation with the progression of the disease. Among the APP, in several cancer groups, C-reactive protein, haptoglobin and alpha 1-acid glycoprotein are more elevated than in controls, and a trend to display higher values is observed in the subjects with metastatic disease; particularly, the frequency of patients with 3 or more proteins above the normal levels is significantly higher in metastatic than in localized cancers (p less than 0.09).