1. Open the chilled, clean oysters according to instructions. Remove them from their shells and put them in the refrigerator or on ice to chill thoroughly.
2. To serve, place one fourth of the celery and parsley in each of 4 large seafood cocktail glasses. Cover with 6 oysters each.
3. Blend the chili sauce, lemon juice, salt and Madeira and stir well. Pour over the oysters.