Ferritin, physiologically an iron-storage protein, has been repeatedly investigated with regard to its role in the development of inflammation and malignancy [2, 3, 6]. The structural heterogeneity of this protein has aroused considerable interest in recent years [2, 3, 5, 6]. Although ferritin is generally regarded as an iron-storage protein, small amounts are also found in the sera of normal individuals, while abnormally high concentrations are found in the serum of patients with malignancies as well as in pregnant women. In the past, considerable attention has also been paid to the structural heterogeneity of ferritin derived from various organs and malignant tissues [3, 5]. It has been reported that the placenta, fetal tissues and malignant tissues contain acidic ferritin, while the liver and the spleen contain ferritin in the basic form. The acidic isoform has been summarized under the term oncofetal ferritin, since these forms share certain physical-chemical characteristics. These so-called oncofetal or placental ferritins (PLF) have repeatedly been shown to have an inhibitory effect on hematopoiesis and T-cell function [2, 6, 7, 10]. The purpose of the present review is to elucidate current data concerning the role of placental isoferritin in pregnancy as well as in the development of breast cancer.