The interfacial potential of cytoplasmic granules, i. e., mitochondria, microsome and of ultra-microsome, was studied by means of classical moving boundary method and Tiselius' electrophoretic method, and the following results were obtained.1. In Miller-Golder's buffer of Γ/2=0.1, the isoelectric-point was about pH 4.5.2. In neutral or alkaline solution, these particles were negatively charged.3. It seems to the author that owing mainly to this negative charge of the particles that cytoplasmic granules, such as microsome and ultramicrosome, can stably exist in cytoplasm, are basophilic, and do not agglutinate when the pH of mediumm is kept alkaline during cell fractionation.4. Judging from the isoelectric points of granules, it may be supposed that there exists about equal amounts of lecithine and cephalin on the surface of granules, however, further experiments are necessary to determine this precisely.