If you read BOOK I, you would not need to ask what juice. Any whole juice will work. Apple Juice and Grape Juice have a longer history than orange juice, but the medical doctors chose the Florida orange, there is no total explaination, I have theorized their reasons more than a few times on this forum. For me, I just trust the author and use the Florida oranges and I consume any and all sweet fruit juices that please me. If Book I was not convincing enough, then read the grape cure and Christopher's 3 day apple juice fast for more clues.