Wow, is this movie a stinker! [SPOILERS]
The effects are laughable, even for back then. Star Trek was doing far better with far less, as was Stanley Kubrick in 2001 about a year or so later.
No explanation whatsoever is given for how the miniaturization process works. Not even a stupid one, or an off-hand one. I know that this is the entire conceit of the movie, which you have to accept in order for there to even be a movie, but you HAVE to give the audience something to go on. They gave us nothing, except that, for some other unexplained reason, nuclear fuel won't miniaturize. Right, because there's a difference between elements that way.
The sets of inside the body just blow. They look completely fake.
There's no light source when they're inside the body. How are we seeing anything at all?
They're supposedly scuba diving in the capillaries. Where are the air bubbles when they breathe out?
Our hero is travelling from the plasma, where everybody's floating (yeah, right), to the lung, where there's air. Where is the plasma/air boundary? Why isn't there a boundary between liquid and gas where they made a hole?
"The Proteus may already be destroyed." So what? Whatever's left is still inside and is going to expand and destroy this guys skull! And even if not - "may be"? Is that really good enough? Yeah, it may be destroyed, so we should just leave it in there, because lord knows, if I'm wrong, it'll just kill this guy who's continued survival is a vital national interest. No big deal.
Then they re-enlarge, and everyone is fine. But, oh wait, didn't we send five guys inside? Where did the fifth guy go? No one seems to give a sh!t about Donald Pleasance. No concern for his life whatsoever.
And finally, they don't even let us know if the guy survived. Isn't this slightly important to the whole plot of the movie? Wow, this is really poor screenplay writing.
I want the doctor to take your picture so I can look at you from inside as well.