Preheat oven to 375 degrees F.
Separate crescent rolls into 8 triangles. Top each crescent with 2 slices of roast beef and one half piece of string cheese.
Roll up and place on baking sheet. Bake for 16 to 18 minutes, or until golden-brown.
While crescents are baking, make Au Jus mix according to package directions. Serve with warm french dip crescents.