A method of separating vesicles whose membrane has been modified with a hydrophilic polymer (polyethylene glycol, etc.). In the method, vesicles (liposome, etc.) differing in the degree of membrane modification are separated by ion-exchange chromatography using a concentration gradient in which the ionic intensity of an eluent is changed with time. Also provided are: a process for producing vesicles purified so as to have a desired degree of modification with a hydrophilic polymer which comprises utilizing the separation method in the step of purifying vesicles a method of evaluating a medicinal vesicle preparation containing a drug therein, the method comprising using the separation method and vesicles in which the membrane has been modified with a hydrophilic polymer and which include no vesicle having a degree of membrane modification less than 0.5 mol%.
