A series of glutathione S-transferase pi (GST-pi) transfectant cell lines have been constructed in activated c-H-ras-transformed NIH-3T3 cells (pT22-3) by using a pKOneo plasmid and an expression vector containing cDNA for GST-pi with a beta-actin gene promoter. From the wild type pT22-3 cells, two clones were selected and designated RGN1 and RGN2. The degree of overexpression of GST-pi was estimated by Northern and Southern blot analysis to be incrementally higher in RGN2 compared with RGN1. Translation of mRNA was estimated by Western blot analysis using isozyme-specific polyclonal antibodies and confirmed the relative GST-pi levels. Each cell line, including the wild type, expressed alpha and mu class isozymes to the same degree and had similar but negligible expression of the mdr 1 gene. Sensitivity to various anticancer drugs and radiation was estimated by a series of cytotoxicity assays. The data confirmed that GST-pi provided a degree of protection against the toxicity of ethacrynic acid and adriamycin, but sensitivity to alkylating agents such as chlorambucil, melphalan, and cis-platinum was not influenced by GST-pi. Similarly, the response to ionizing radiation was similar for each line. Since the levels of intracellular GSH were also not significantly different, the availability of co-substrate was not a factor in determining response. In creating the GST-pi transfectants, these data establish that while increased isozyme levels can play a role in determining sensitivity to some agents, the protective effect is selective.