Nonselenium glutathione peroxidase (NSGP) is a new member of the antioxidant family. Using antibodies to recombinant NSGP we have examined the distribution of this enzyme in normal, Parkinson's disease (PD), and dementia with Lewy body disease (DLB) brains. We have also co-localized this enzyme with alpha-synuclein as a marker for Lewy bodies. In normal brains there was a very low level of NSGP staining in astrocytes. In PD and DLB there were increases in the number and staining intensity of NSGP-positive astrocytes in both gray and white matter. Cell counting of NSGP cells in PD and DLB frontal and cingulated cortices indicated there was 10 to 15 times more positive cells in gray matter and three times more positive cells in white matter than in control cortices. Some neurons were positive for both alpha-synuclein and NSGP in PD and DLB, and double staining indicated that NSGP neurons contained either diffuse cytoplasmic alpha-synuclein deposits or Lewy bodies. In concentric Lewy bodies, alpha-synuclein staining was peripheral whereas NSGP staining was confined to the central core. Immunoprecipitation indicated there was direct interaction between alpha-synuclein and NSGP. These results suggest oxidative stress conditions exist in PD and DLB and that certain cells have responded by up-regulating this novel antioxidant enzyme.