Medium rare prime rib‚ that is, meat that is mostly pink with a deeper, nearly red center, is cooked to about 130-135°F. At 120-125°F the meat is rare, and at 140-145°F, it is well done. You should know the USDA recommends cooking beef to a minimum of 145°F. But I, like most people, don't want my steaks or beautiful roasts well-done. When I make beef or lamb roasts, I serve them at 130°F, which means I take them out of the oven at 125-130°F.
Degrees of doneness for roasts and steaks:
Rare 120-125°F - Red interior which may fade to pink at the outermost edges
Medium Rare 130-135°F - Pink with a deeper, nearly red center
Medium 135-140°F - Mauve, uniformly colored interior
Well-done 145°F+ - Brown, no pink
Recipe: Classic Prime Rib