I don't think this was my first Superman/Batman team-up book but it has certainly remained stuck in my memory for decades. I think I read this (repeatedly) between the ages of 8 and 10 until it was lost to the winds of fate. I recently found a copy in the cheap bins and was thrilled to see that - as silly as it is - it holds up as a fun little story.
Of course, at the time I had no idea that scripter Bob Haney was blatantly ripping off the film FANTASTIC VOYAGE (1968) and crossing it with a monster story. It's just such barely concealed theft that makes returning to the comic books of my youth so much fun. I got the occasional title steal from classic literature and the often over-obvious character names for both heroes and villains but lifts from things I had yet to be exposed to went past me easily. Now, spotting this stuff is part of the joy of rediscovery when I find one of these pieces of my childhood. Of course, the fact that The Atom is reduced to dust and then injected into a human body where he reassembles himself to do battle with gremlin-like 'germs' has me smiling the whole time. This kind of madness is why the 1960's and 70's DC comics were seen as so unhinged.
And, as a side note, Dick Dillin's artwork for this issue is amazing! I always associate him with his very long run on the original Justice League series and his pencils here show his style off well.