Amyloid fibrils from an individual with heredofamilial amyloidosis were found to be composed of plasma prealbumin. To study this protein a three step procedure to isolate prealbumin from plasma was developed. It entailed ion exchange chromatography on DEAE Sephadex, affinity chromatography on Affi-gel Blue and gel filtration on AcA-34. Trypsin Digests of prealbumin were separated by reverse phase HPLC and the pattern compared to that from the normal protein. Only one unexpected peptide was found and it represented the substitution of a methionine for a valine at position 30 in the molecule. This substitution accounts for about 1/3 of the isolated molecules and it represents the first point mutation identified in human plasma prealbumin.