Ferritins are iron-containing proteins found in normal tissues; they increase in concentration in many tumors and the blood of tumor-bearing individuals. We utilized a double-antibody radioimmunoassay for measurement of serum ferritin and defined the upper limit of normal as 146 ng/ml for women (mean 34 ng/ml) and 193 ng/ml for men (mean 93 ng/ml). Serum ferritin levels exceeded these limits in preoperative sera of 41% of women with mammary carcinoma (mean 199 ng/ml) and in 67% of women with locally recurrent or metastatic mammary carcinoma (mean 671 ng/ml). Individuals with hepatic inflammatory states are known to have high serum ferritin, and ferritin was increased in 43% of patients with hepatitis or cirrhosis (mean 364 ng/ml) and in 13% of patients with ulcerative colitis or gastroduodenal ulcers (mean 106 ng/ml). Measurement of serum ferritin may be useful in evaluation of patients with breast cancer and in monitoring their response to therapy.