The wide-spread use of and demand for myelin basic protein for immunologic studies has prompted us to re-examine the details of its isolation from CNS tissue of various species. The procedure described in this communication for the isolation and purification of myelin basic protein does not require column chromatography and is therefore suitable for large scale preparation of a reasonably pure product with simple laboratory equipment. If certain precautions are taken, the yield and quality of the product are reproducible. Certain contaminants which may accompany myelin basic protein during purification by procedures currently in use are pointed out, and their possible influence on the immunologic behavior of myelin basic protein is discussed. Suitable electrophoretic techniques for the detection of these contaminants as well as details for their removal from the myelin basic protein are described.