Scald milk in a large Dutch oven. Combine melted butter and flour, stirring until smooth; add to hot milk, stirring well. Add celery, and cook over medium heat 10 minutes, stirring constantly. (Do not boil.) Stir in salt, white pepper, and red pepper, if desired.
Add oysters and liquid, stirring well. Reduce heat to low, and continue cooking, stirring constantly, until mixture is thoroughly heated and oyster edges curl. Ladle into individual soup bowls. Serve immediately.