Studies have suggested that the α class glutathione S-transferase (GST) may protect cells from the chemotherapeutic drugs chlorambucil and melphalan. In order to further define the function of human α class GST, a complementary DNA which encodes it was ligated into an expression vector under the direction of the human metallothionein-IIA promoter and stably transfected into human MCF-7 breast cancer cells in conjunction with the G418-selectable plasmid pSV2neo. Clonal cell lines were identified which expressed increased levels of GST enzyme activity (2.2- to 5.6-fold). The transfected cell lines also had increased peroxidase activity using cumene hydroperoxide as the substrate (1.9- to 3.8-fold) which is consistent with the intrinsic peroxidase activity of α class GSTs. Southern blot analysis indicated that genomic DNA from these cells contained a fragment indistinguishable from the transfected α class GST complementary DNA (850 base pairs); Northern blot analysis of total cellular RNA indicated that these cells contained appropriately sized α class GST RNA (980 nucleotides); and Western blot analysis indicated that, while MCF-7 cells contained no detectable α class GST protein, the transfected cells contained markedly elevated levels of α class GST but no detectable µ or π class GST. These α class GST transfected cells had increased resistance to ethacrynic acid (2.1- to 3.0-fold). However, the transfected cells failed to show any increased resistance measured at the drug dosage which inhibited 50% of the colony formation to the chemotherapeutic drugs chlorambucil, melphalan, Adriamycin, or cisplatin under conditions of either continuous or 1-h drug exposure. Neither was there any change in sensitivity to the cytotoxins benzo(a)pyrene, benzo(a)pyrene-trans-7,8-dihydrodiol-9,10-epoxide (anti), or 1-chloro-2,4-dinitrobenzene. These studies indicate that expression of this human α class GST by itself in MCF-7 human breast cancer cells does not confer resistance to the chemotherapeutic drugs tested under the conditions used in these studies.