1. Basic protein (mol.wt. 16500) and polypeptides (mol.wt. 3500) were isolated from bovine spinal cord by a procedure involving defatting, acid extraction of the defatted material and repeated chromatography on Sephadex G-50. Similar fractions were isolated from guinea-pig brain. 2. These fractions produced experimental allergic encephalomyelitis in guinea pigs. 3. The polypeptides appeared to be derived from a basic protein of myelin as a result of the action of an acid proteinase during extraction with acid. Similar proteolysis might also occur in the isolation of other biologically active polypeptides from acetone-dried powders of nervous tissue. The activity of the acid proteinase was lowered by defatting with chloroform-methanol. 4. Peptides from tryptic digests of encephalitogenic polypeptides and protein were also encephalitogenic, which suggests that the encephalitogenic determinant may be quite a short sequence of amino acids. 5. These encephalitogenic polypeptides are further examples of antigens of low molecular weight.