Using differential display, a cDNA fragment was identified as being overexpressed in a mouse lymphoma cell line that had gained resistance to cell death after exposure to a variety of agents used in cancer therapy. The full-length cDNA of 1.1 kb that was cloned contained an open reading frame coding for a previously unidentified 28-kDa mammalian protein, p28. p28 showed significant homologies to a large family of stress response proteins that contain a glutathione S-transferase (GST) domain. In correspondence with the sequence homology, p28 was found to bind glutathione; however, GST or glutathione peroxidase activity could not be demonstrated. Northern analysis of the mRNA of this protein showed abundant expression in mouse heart and liver tissues, whereas anti-p28 antibody binding identified p28 expression in mouse 3T3 cells and early passage mouse embryo fibroblasts. Subcellular protein fractionation revealed p28 localization in the cytoplasm, but with thermal stress p28 relocated to the nuclear fraction of cellular proteins. Based on sequence homology and protein activity we conclude that p28 acts as a small stress response protein, likely involved in cellular redox homeostasis, and belongs to a family of GST-like proteins related to class theta GSTs.