Melt butter in a heavy saucepan over low heat; add celery, and cook 5 minutes or until tender. Add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly. Add turkey, salt, and pepper to white sauce, stirring well.
Spoon mixture into a lightly greased 1-quart casserole; top with breadcrumbs. Bake at 400° for 30 minutes or until breadcrumbs are browned.