Concentrations of copper, zinc and magnesium were measured in serum and in malignant and non-malignant tissue specimens from patients with carcinoma of the breast, stomach and colon. The copper concentrations of cancerous tissues were approximately 20% lower than in normal tissue from the same patients. In specimens of sera the mean copper concentrations in the 3 groups of patients were increased by 31-40% compared with healthy subjects. Zinc concentrations were increased in malignant tissues and decreased in the serum. There were no significant changes to magnesium levels in serum or tissues. The results suggest that movements of copper out from, and of zinc into, tissues occur and it is proposed that these changes could be a response to enhanced cytokine production rather than being a feature of any mechanism(s) for the onset of cancer.