Metallothioneins (MTs), a group of cysteine-rich proteins with a small molecular mass, are known to have metalloregulatory functions. MT gene expression has been demonstrated to be cell type-specific and differentially regulated (possibly related to their germ layer origin and different functional states). In vitro studies suggest that MT-2A, MT-IE, and MT-1F isoforms may be related to breast cancer. In this study, data on MT-2A, MT-1E, MT-1F mRNA analysis via reverse transcription-polymerase chain reaction in invasive ductal breast cancer tissues and their adjacent benign breast tissues from 27 mastectomies are presented. Expression of mRNA in all the three MT isoforms was detected in both cancerous and adjacent benign breast tissues (with MT-2A mRNA expression being the highest). MT-1F expression was significantly higher in benign breast tissues compared with the breast cancers (P=0.017). In situ hybridization confirmed the expression of MT-2A mRNA in the myoepithelial cells of the breast tissues. Immunohistochemical localization of the MT protein revealed that myoepithelial cells consistently expressed the MT protein, while the cancer cells expressed MT with great variation. Based on our immunohistochemical and mRNA analysis, it is likely that the three MT isoforms are specifically expressed in myoepithelial cells of benign breast tissues and cancer cells of the invasive ductal breast cancer tissues. As MT expression occurs in myoepithelial cells and ductal breast cancer cells, our finding supports the proposition that loss of myoepithelial cells in invasive mammary cancers may be compensated in part by changes in the tumor cells, which may subsequently be the basis for studying the role of MT in breast physiology and carcinogenesis. Differential MT-1F expression in breast myoepithelial cells warrants further study.