PREVIOUS reports have described the preparation of proteins from bovine spinal cord, which, when injected into guinea pigs, caused allergic encephalomyelitis ( KIES, ROBOZ and ALVORD, 1956; ROBOZ, HENDERSON and KIES, 1958~). One of these proteins was isolated in a purified form and shown to be homogeneous by ultracentrifugal and electrophoretic measurements (ROBOZ, HENDERSON and KIES, 19586; MACKENZIE, 1958). It was classified, according to its amino acid composition, as a collagen-like protein (ROBOZ, HENDERSON and KIES, 19586). Since its activity accounted for only a small part of the total activity in the spinal cord other methods of extraction were tried (Ronoz and HENDERSON, 1959). Potassium chloride and sodium citrate solutions were used as extraction media, and active protein preparations were obtained. The potassium chloride extract contained about nine or ten proteins as shown by strip electrophoresis. The fractionation of these proteins by the use of ion exchange and continuous preparative electrophoresis methods is the subject of these studies. The work has not been reported previously except for a short account by ROBOZ EINSTEIN, ROBERTSON and DICAPRIO (1960). E X P E R I M E N T A LFor the preparation of the protein extract, bovine spinal cord was used. (This was generously supplied by a local slaughter house, James Allen & Sons, San Francisco.) After removal of the dura mater and blood vessels, the cord was homogenized in a Waring blender at 4'. The homogenized cord was lyophilized, pulverized, and treated with acetone and petrol-ether precooled in dry ice. This procedure resulted in partial removal of the lipids, thereby making the subsequent filtration and centrifugation easier. Treatment with distilled water or sodium acetate solution followed, 1 1. being used to every 100 g of tissue for 24 hr at 4" under constant stirring with a large-bladed propeller, driven at low speed to minimize emulsification and foaming. The water and the sodium acetate extracts had been found in earlier experiments t o be inactive and were not used in these investigations. From the residue the proteins were extracted overnight at 4" under constant stirring with 5 ", KCI in a ratio of 1 I. to every 100 g of fresh tissue. This was centrifuged under refrigeration at 8000 rev/min (10,400 g) and the supernatant solution further clarified by filtration. The KCI extracts were dialysed against dilute NaCl (0.01 M) and brought to 40% saturation with (NH,),SO,. Since the resulting precipitate was inactive, it was discarded. The supernatant liquor was fully saturated with (NH,),SO, and left overnight a t 4 . A flocculent suspension of proteins was formed and decanted