The method involves guiding whole blood via a hollow fiber membrane-filter unit (3) under flow of gravitational force to obtain two fractions such as cell-free plasma fraction and erythrocyte/plasma-suspension. The fractions are re-guided to an inlet of the filter unit in a recirculation manner to pass the fractions once in a prolonged manner. A substance that changes cellular characteristics of the erythrocyte/plasma-suspension in a desired manner is added to the erythrocyte/plasma-suspension, before passage of the hollow fiber membrane-filter unit. An independent claim is also included for a device for implementing the method for separation of whole blood into fractions.
