A method and device for blood treatment involving infusion of replacement fluid are provided. In an embodiment, fluid is passed either by pump or passively by gravity feed, through a microporous sterilization filter (11) from a fluid source to a replacement fluid container (1). The latter forms a batch that may be used during treatment. The advantage of forming the batch before treatment is that the rate of filtering need not match the rate of consumption during treatment. As a result, the sterilization filter (11) can have a small capacity. In another embodiment, a filter (11) is placed immediately prior to the point at which replacement fluid is injected into the patient's bloodstream. The latter may be used in combination with the former embodiment as a last-chance guarantee of sterility and/or that the fluid is free of air bubbles. It may also be used as the primary means of sterile-filtration.