A patient is presented in whom the amyloid component of an intracerebral 'amyloidoma' has been purified and characterized by amino acid sequence analysis. The material originated from an autopsy of a 76-year-old man who 15 years earlier had been operated for an intracerebral 'amyloid tumour'. The tumour had recurred and grown to an almost walnut-sized mass in the right cerebral hemisphere. It was located in the parietal lobe close to the lateral ventricle and had a close connection to the choroid plexus. Histological examination showed large masses of amyloid surrounded by some plasma cells and a few macrophages of the foreign body type. Amino acid sequence analysis of a major fibril subunit protein showed homology with the variable region of a monoclonal lambda immunoglobulin light chain, subgroup III or IV. This shows that the amyloid in the 'tumour' was of AL type and presumably derived from local synthesis by plasma cells.