The occurrence of circulating immune complexes (CIC) was investigated in serum samples from 139 patients with breast cancer, using the 125I-C1q binding test. In the 85 cases of regional forms, the level of Clq binding activity did not appear to be correlated with the clinical TN stage, nor with the local treatment of the tumor. Conversely it was clearly elevated in the 13 cases of inflammatory breast tumors, and the decrease of CIC at the post-chemotherapy, but pre-surgical stage may involve the inflammatory signs in this augmentation of CIC. Frequency of occurrence and levels of CIC were slightly increased in the 41 metastatic breast cancer patients when compared to the cases of local breast carcinoma. In the disseminated forms, increase, no change, or decrease of CIC levels showed a poor correlation to progression, stabilisation, or improvement of the disease. CIC seems to be of slight prognostic value in breast cancer with local forms, but regarding progression of the disease, metastatic breast cancer patients with elevated CIC did not prove to be unfavorable group. So far, CIC did not appear to be a tumor marker or an evident prognostic factor of clinical relevance in breast cancer. Assays for antigen-specific CIC might be of greater clinical significance, in breast cancer, than the antigen non specific assays available at present.