If the leak is from the drain plug, see if it's loose. If the leak is from a metal part of the radiator, take it to a specialist who will remove the radiator, solder it, and reinstall it. If the leak is from an end tank -- the top or bottom portions of the radiator -- and the end tank is plastic, time to say goodbye to the radiator. If the leak is a hose, replace it with the correct hose plus new clamps. Sometimes the leak is from the radiator's fitting to the hose; in this case the radiator also needs replacement.