The values of circulating immune complexes (CIC) in patients with breast cancer were followed for one year at 4-month intervals by means of the "PEGIKEM" test. From the total of 67 patients, 37 were treated surgically and a significant decrease in mean values of CIC occurred 8 months after the operation. 30 patients with a more advanced stage of the disease treated with radiotherapy and chemotherapy showed a decrease in mean CIC values already after 4 months but later on the CIC values increased again. No significant differences in the mean CIC values prior to treatment between individual clinical stages were found, only a certain tendency towards a more frequent occurrence of higher CIC values in more advanced stages was noted. Nor were the findings in lymph nodes reflected in the mean CIC values prior to treatment. Comparison of CIC values prior to treatment and the results of 2-hour blood sedimentation, revealed conformity in only 40% of the patients. In the other cases higher values of blood sedimentation were more frequent. The study of prognostic value of CIC after 3 and 5 years follow-up is in progress.