The development of multidrug resistance in MCF7 human breast cancer cells is associated with overexpression of P-glycoprotein, changes in activities of several detoxication enzymes, and loss of hormone sensitivity and estrogen receptors (ERs). We have cloned the cDNA for one of the drug-detoxifying enzymes overexpressed in multidrug-resistant MCF7 cells (AdrR MCF7), the anionic isozyme of glutathione S-transferase (GST pi). Hybridization with this GST pi cDNA, GST pi-1, demonstrated that increased GST pi activity in AdrR MCF7 cells is associated with overexpression but not with amplification of the gene. We mapped the GST pi gene to human chromosome 11q13 by in situ hybridization. Since multidrug resistance and GST pi overexpression are associated with the loss of ERs in AdrR MCF7 cells, we examined several other breast cancer cell lines that were not selected for drug resistance. In each of these cell lines we found an inverse association between GST pi expression and ER content. We also examined RNA from 21 primary breast cancers and found a similar association between GST pi expression and ER content in vivo. GST pi mRNA content in 11 ER-positive tumors (less than or equal to 10 fmol/mg of protein) was significantly different from the GST pi content of 10 ER-negative tumors (P = 0.002; Mann-Whitney Wilcoxon test for two independent samples). The finding of similar patterns of expression of a drug-detoxifying enzyme and of ERs in vitro as well as in vivo suggests that ER-negative breast cancer cells may have greater protection against antineoplastic agents conferred by GST pi than ER-positive tumors.