Placental isoferritin (PLF), an acidic isoform of ferritin, and its unique superheavy chain p43 have been recently described to be synthesized by breast cancer cell lines but not by normal breast epithelial cells. Since previous reports have demonstrated a correlation between the content of 'normal' ferritin in breast cancer tissue, degree of differentiation, and prognosis, we have tried to evaluate the correlation of p43 in the cytosol of 122 breast cancer samples with commonly applied prognostic factors and features of proliferation and differentiation. In parallel, we investigated the correlation of p43 expression in MCF-7 and T47-D breast cancer cell lines during proliferation induced by estradiol plus fetal calf serum (assessed by 3H-thymidine incorporation), compared to p43 expression in stationary non-stimulated cell cultures. The levels of p43 in breast cancer cytosols correlated significantly negatively with tumor size (p = 0.0001), histologic grading (p = 0.0038), nuclear pleomorphism (p = 0.0019), rate of mitosis (p = 0.0002), and lymphocytic reaction (p = 0.0001), and significantly directly with the estrogen receptor status (p = 0.0009). Although patients with a higher p43 content showed a trend for a better outcome (median follow-up: 61.4 months), an independent influence of the cytosolic p43 content on survival could not be confirmed by a multivariate Cox model. In accordance with the observed negative correlation of features of differentiation vs. p43 expression, induction of proliferation by estradiol plus FCS added to serum-free tissue culture medium correlated with a decrease of p43 synthesis in both cell lines. Expression of p43 in estrogen and FCS-absent media revealed also a decrease in relation to a low spontaneous proliferation. However, the drop of p43 synthesis was significantly stronger in cell lines with estrogen-stimulated proliferation. Our in vitro and cytosol results confirm recent clinical observations describing an inverse correlation of p43 synthesis with the degree of proliferation and differentiation in breast cancer. However, the pathologic mechanisms leading to this phenomenon as well as the negative correlation with lymphocytic infiltration are still unclear and need to be further elucidated.