Metallothioneins (MTs) belong to a family of cysteine-rich, metal-binding intracellular proteins, which have been linked with cell proliferation. In this study, expression levels of the 8 known MT-1 and MT-2 functional isoforms in human invasive ductal breast cancer specimens were determined by RT-PCR. The expression profiles of the MT protein and MT-2A mRNA were further evaluated in 79 cases of human invasive ductal breast carcinoma by immunohistochemistry and in situ hybridization, and correlated with cancer cell proliferation (determined by Ki-67 nuclear antigen immunolabeling). MT-1A, MT-1E, MT-1F, MT-1G, MT-1H, MT-1X and MT-2A but not MT-1B, were detected in breast cancer tissue samples. The MT-2A mRNA transcript was the highest among all the isoforms detected. A positive correlation was observed between MT-2A mRNA and MT protein expression with Ki-67 labeling (P = 0.0003 and P < 0.0001, respectively) but not with apoptosis (P = 0.1244 and P = 0.8189, respectively). Co-localization of the MT protein and Ki-67 nuclear antigen in breast cancer cells was demonstrated by double immunofluorescence staining. There was also significantly higher MT protein and MT-2A mRNA expression in histological grade 3 tumors than in histological grade 1 and 2 tumors. The finding that MT 2A appears to be the main isoform associated with cell proliferation in invasive ductal breast cancer tissues, may have therapeutic implications.