Ferritin concentration was measured in cytosol extracts of 44 mammary carcinomas and 14 benign breast tissues. A six-fold difference was observed (mean, 364.6 +/- 223.3 ng/mcp in malignant tissue versus mean, 60.2 +/- 42.1 ng/mcp in benign tissue P less than 0.001). Thirty-five malignant tissue specimens were reviewed independently by a pathologist without knowledge of their ferritin contents. Higher concentrations of ferritin were present in malignancies with greater degrees of epithelial proliferation and plemorphism suggesting the malignant epithelium as the major site of the increased ferritin. There was no correlation between desmoplastic reaction within the tumors or inflammation within or adjacent to the tumors and ferritin concentration. Ferritin in breast tissue may be important as a marker of neoplasia, a source of elevated serum ferritin, an indicator of clinical prognosis or an immunosuppressive substance.