Amyloid was extracted from the spleen of a patient with primary amyloidosis by homogenizing it at high speed with water after preliminary treatments, first to remove proteins soluble in saline, and then to remove salts. The extracts containing amyloid appeared to be clear at concentrations up to 6 mg/ml of protein. The material gave little sediment on being centrifuged up to 20,000 g for 1 hr, but the protein was sedimented at 100,000 g in 1 hr. The amyloid could be precipitated from the extracts by addition of NaCl to 0.0075 mole/liter or of CaCl(2) to 0.0025 mole/liter. The protein-bound Congo red formed a red precipitate and this property was used to estimate recovery and purity of amyloid during extraction. On electronmicroscopy the isolated amyloid proved to be morphologically pure. It existed either as single filaments measuring 60-80 A in diameter or as large aggregates of these filaments.Freshly isolated amyloid in water sedimented as a single homogeneous peak with an s degrees (20,[unk]) of about 45-50S. On standing, the solution became cloudy and more rapidly sedimenting components appeared. On electrophoresis the material migrated as a homogeneous peak towards the anode. The protein had an amino acid composition different from that of all known serum proteins. It was rich in acidic amino acids and had little cysteine and methionine and no hydroxyproline. The total content of carbohydrate was less than 2%.