The average ceruloplasmin levels of 29 patients with active breast cancer and 22 patients in remission were 824 +/- 61 mg/l and 630 +/- 18 mg/l respectively. The average ceruloplasmin level of 17 patients with benign breast diseases was 555 +/- 29 mg/l and of 18 healthy women in a control group 584 +/- 17 mg/l. Breast cancer patients not in remission had ceruloplasmin levels which were significantly increased when compared to the other 3 groups. The CA 15-3 levels and ceruloplasmin levels were positively correlated. We propose that ceruloplasmin may be used as a tumour marker in the follow-up of patients with breast cancer.