A novel anti-polyethylene glycol monoclonal antibody and its preparation are disclosed. Such an antibody can be used for determining polyethylene glycol concentration in vitro or accelerating the clearance of a polyethylene glycol containing compound from the blood circulation in the human body thereby reducing the toxicity associated with the polyethylene glycol containing conjugate. The antibody is particularly useful in cancer therapy where the therapeutic agent is selectively delivered to the tumor by increasing the tumor/blood ratio of the polyethylene glycol containing compound.