On the same DVD as Dr. X is the sequel...
This film has a great cast as well as an engaging and rather unique plot. In short, people with a certain blood type are all being bumped off in the same way. Could you find a more ultra-specific premise? OMG! I'm Type A!!! RUN!!!
The real treat is the performances. Humphrey Bogart is in a very atypical role as the creepy "assistant" to blood researcher Dr. Flegg, played by John Litel. It's fascinating to see Bogart play this awkward, geeky character. His performance is so understated and completely authentic; you get the sense that this major star was a pretty down-to-earth guy with a deep understanding of all kinds of people.
I also loved John Litel's Dr. Flegg. At one point in the film, he very methodically demonstrates his signature medical procedure, which is so far-fetched and outlandish, but he pulls it off with such matter-of-fact conviction. It's some darn great acting and it's pretty hilarious.