The Tiselius technique of electrophoresis has been used to study the homogeneity and mobility of pituitary lactogenic hormone. The lactogenic preparation employed by us shows only one sharp band in schlieren photographs, indicating a high degree of homogeneity. From the determination of the mobility of the hormone at different hydrogen ion concentrations in acetate and phosphate buffer of ionic strength 0.055, the isoelectric point was found to be 5.70 and the See PDF for Equation value 4.5 x 10(-5). The difference in the mobility reported for White's crystalline preparation and that found with our high potency lactogenic preparation is discussed.