Substantial amounts of a single protein have been extracted into electrophoresis sample buffer from archived formalin-fixed brain blood vessels, taken from a case of cerebral amyloidosis. Cyanogen bromide cleavage and tryptic digestion of the protein on Western blots allowed amino acid sequences from three resultant peptides to be determined. Comparison of these peptides with database sequences identified the extracted protein as being derived from an immunoglobulin light chain. This is the first demonstration of amino acid sequencing of a polypeptide extracted from formalin-fixed tissue. This case also appears to be unique, since primary cerebrovascular amyloidosis involving immunoglobulin light chains has not been previously described. The amyloid protein had clearly resisted formalin fixation; it is possible that this resistance occurred because the protein was deposited in large amounts as insoluble densely packed aggregates, which may exclude infiltration of the formalin. This technique may therefore have applications in the post-mortem diagnosis of amyloidoses and in the purification of other amyloids.