Photographic sensitive layer should produce sufficient density with the least layer thickness. In order to satisf y this demand, gelatin must retain as much silver halide as possible dispersed in the emulsion. The limit of silver halide, dispersed colloidally without sedimentation, in a given quantity of gelatin may vary with gelatins. The authors propose “colloidal retentivity” as a measure of this protective colloid power of gelatin against silver halide in the photographic emulsion. Colloidal retentivity is so defined as to show 100% when emulsion grains are well dispersed without sedimentation, while the more sedimentation, the les colloidal retentivity. Two methods of the measurment of colloidal retentivity are introduced. Colloidal retentivity as depending on the ripening temperature, rotation speed of a stirrer, silverhalidegelatin ratio and gelatin characteristics are studied. Colloidal retentivity brings different information about the ripening as compared with the turbidity measurment. Colloidal retentivity during the ripening passes maximum and minimum points while turbidity increases monotonously. The graingrowth mechanism is discussed on these results.