BUTTERMILK SALAD DRESSING

2 cups buttermilk
2 cups mayonnaise
1 tablespoon onion powder
1 tablespoon dried parsley flakes
1-1/2 teaspoons garlic powder
1/2 teaspoon salt
1/2 teaspoon celery salt
1/4 teaspoon pepper

In a large bowl, whisk all ingredients until smooth.  Cover and refrigerate until serving.