Expression of metallothionein (MT) isoforms by a human breast cancer cell line, PMC42, which retains many characteristics of normal breast epithelial cells and expresses functional estrogen receptors, was examined because it has been proposed that human breast cancer cells which are estrogen receptor positive can be differentiated from those which are estrogen receptor negative, by failure to express MT-1E [J.A. Friedline, S.H. Garrett, S. Somji, J.H. Todd, D. A. Sens, Differential expression of the MT-1E gene in estrogen-receptor positive and -negative breast cancer cell lines, Am. J. Pathol. 152 (1998) 23-27]. Using RT-PCR, PMC42 cells were found to transcribe genes for the MT isoforms IE, IX and 2A but not 1A or 1H. In order to examine which of the expressed isoforms might protect against metal toxicity, the cells were challenged with high concentrations of zinc and copper. Using competitive RT-PCR, cells resistant to 500 microM zinc showed 7+/-2 fold (SD, n=3) increases in expression of MT-1X and 6+/-3 fold increases in expression of MT-2A compared to control cells in normal media. For cells resistant to 250 microM copper the corresponding increases were 37+/-13 and 60+/-20 fold, whilst for control cells treated with 250 microM copper for only 6 h, increases were 10+/-3 and 6+/-3 fold. There was only a low level of expression of MT-1E in untreated cells and but a >120 fold increase in copper- resistant cells. Thus estrogen receptor positive cells cannot, in general, be differentiated from estrogen receptor negative cells by failure to express MT-1E, as suggested by Friedline et al. (1998). Increased expression of MT-1E, as well as MT-1X and MT-2A, protects against metal toxicity in PMC42 breast cancer cells.