Effects of electromagnetic fields (EMFs) on the incidence of breast cancer (BC) have been proposed by a number of epidemiological studies. The molecular mechanism of the impact of EMFs on cells is not yet clear, although changes in gene expression have been reported in various cellular systems. In this investigation, the interference of low-frequency EMFs with the plasminogen activator system was examined in BC cells.MCF-7 BC cells from 2 different sources were exposed to highly homogeneous 50-Hz EMFs. Changes in gene expression were analyzed by reverse transcriptase-polymerase chain reaction.In MCF-7 cells exposed to 1.2 microT EMF expression of the urokinase plasminogen activator gene and of plasminogen-activator inhibitor-1 was markedly increased. The expression of the receptor for urokinase plasminogen activator was only marginally increased in 1 of the 2 tested cell lines and expression of the tissue plasminogen activator was at least slightly down-regulated in BC cells exposed to EMFs.EMFs may be able to increase the metastatic potential of breast tumors. The use of our newly established exposure system for EMFs may allow us to study the signaling processes involved in the induction of a metastatic phenotype of breast cancer cells.