Fantastic Voyage (1966) on Netflix

The battle being waged by the super powers is who can perfect miniaturization. Now both sides are stumped as to how to control it. So far they can miniaturize anything but up to 60 minutes only. A scientist who is believed to have solved it, was about to give it to the Americans when an attempt is made on his life. Fortunately he is not dead but he is in a coma and any conventional means to operate could be fatal. So a plan is made to miniaturize a vessel and a team of doctors send them into the man's body and that they will go to the damaged area and fix it. Problem is that there is a report that someone on the team could be working for the other side. So they send Grant, a government agent to make sure everything goes OK.