No evidence is found to support the theory that a carbohydrate is the factor responsible for the Marchi reaction, although a polysaccharide may be present in myelin. The solubility of Marchi material in petroleum ether and cold acetone, together with its insolubility in water, suggests that it is a lipid. Cholesterol is the myelin lipid whose solubility properties most closely resemble those of Marchi material.