Circulating immune-complex (IC) levels of patients with breast cancer were measured before and after surgery using a complement-consumption technique. 16 of 26 patients showed raised pre-operative IC values. In 15 out of 16 patients the pathological IC levels decreased within 2–10 days following surgery. Suggestions about the applicability of this test not only for prognosis, but also for monitoring the course of breast cancer need to be confirmed by further investigations.