A specific mutation (A53T) in the encoding region for α-synuclein has been identified in a large multigenerational family with an autosomal dominant parkinsonism known as the Contursi kindred. In this study, we used a monoclonal antibody directed against α-synuclein in order to identify novel proteins in the brain of an affected member of this kindred who had come to autopsy. Homogenates from the frontal cortex and caudate nucleus were examined using Western blot techniques and compared to matched autopsy specimens from control subjects and patients with various forms of parkinsonism. Western blots, using a 15-min exposure time, revealed the expected 19-kDa band representing α-synuclein in all brain samples examined. However, a novel band in the 36-kDa range was also present in the Contursi brain which was not seen in cortex or caudate from control brains or in frontal cortex from 14 cases of typical Parkinson's disease. With a 24-h exposure time, this band was faintly seen in the caudate nucleus of three of the Parkinson's disease cases. Surprisingly, the 36-kDa band (as well as other high-molecular-weight bands) was also present in frontal cortex and caudate nucleus in 3 additional cases that met diagnostic criteria for both Parkinson's disease and Alzheimer's disease. A preliminary analysis of samples from the frontal cortex of 10 Alzheimer's disease cases revealed a 36-kDa band in only one instance. The identification of novel α-synuclein-immunoreactive bands in these various forms of parkinsonism may open new research avenues for exploring the relationship between abnormal protein deposition in the brain and one or more neurodegenerative disorders, including the Contursi form of familial parkinsonism.