Methods are provided for anticoagulating blood. Whole blood is drawn from a donor into a system at a draw flow rate. Anticoagulant from an anticoagulant source is pumped into the system at an anticoagulant flow rate to mix with the blood. The anticoagulated blood may be subsequently processed in any of a number of known ways, including separating it and removing at least a portion of one of the components of the blood. Thereafter, at least a portion of the remaining blood may be returned to the donor. The anticoagulant flow rate is independent of the draw flow rate and can be based on a number of factors, including the weight of the donor and the rate at which the donor can metabolize the anticoagulant.