Abstract Tissue and cultured cells of different species and embryological origins incubated with serum (diluted up to 10 000-fold) or cerebrospinal fluid (CSF) (6-fold dilution) from a 48-year-old female patient with idiopathic late-onset cerebellar ataxia, exhibited a bright specific perinuclear staining when studied by indirect immunofluorescence microscopy. The pattern of the staining was that characteristic of the Golgi apparatus, consisting of a crescent-shaped juxtanuclear reticulum located in the vicinity of the microtubule organizing center. Changes in location and organization of the organelle stained by the patient's serum during mitosis or after incubation of the cells with Colcemid, taxol or monensin, resulted in a disruption of the reticulum that followed the expected patterns for Golgi apparatus. The staining was specifically absorbed with Golgi cisternae-enriched membrane fractions. Finally, dot-immunoblotting studies of membrane and soluble fractions of Golgi cisternae and vesicles showed that the anti-Golgi antibody (AGA) reacted with the cytoplasmic domain of an integral membrane protein contained in the Golgi cisternae. The presence of this unusual autoantibody in an idiopathic late-onset cerebellar ataxia-bearing patient can afford some insights into the pathogenesis of these neurological diseases.