The moving boundary method of electrophoresis has been applied to a study of pituitary extracts and purified protein fractions derived from these extracts. The technique employed was that developed by Tiselius and involved the optical observation of the protein boundaries by Toepler's schlieren method. The present experiments were designed primarily to determine the number of proteins present, the degree of homogeneity of the various fractions, and the electrophoretic mobility of the individual components under standard conditions. The preparations studied included crude gland extracts obtained with dilute alkali, glycerol, and saline; purified pituitary fractions prepared by isoelectric and precipitation procedures; and freshly prepared and aged solutions of crystalline prolactin. The bulk of the crude gland extracts is composed of physiologically inert proteins, the gradual removal of which in the course of the chemical purification procedures could be controlled by electrophoretic analysis. Freshly prepared solutions of crystalline prolactin exhibit a high degree of electrochemical homogeneity. Upon storage, however, a second component, presumably denatured prolactin, is formed.