The Quatermass Xperiment (1955 UK) (The Creeping Unknown)

This predates THE BLOB (1958) as one of those creepy alien things from space invading us/infecting us. It's a condensed version of the TV BBC production from 1953. This is also the first film in the Quatermass trilogy; it gets a bit confusing.

The message of these films, especially in this first one, was that, yes, it's understandable that mankind progresses in its scientific inquiries, such as taking the first steps towards exploration of outer space, but we all better be ready for the inevitable drawbacks or, uh, fallout, so to speak. At the start of the film, one of our rocketships crashlands to Earth, ending up in a farmer's field.  The experiment is over at this early stage of the film; now begin the negative aspects. Of the 3 astronauts, only one stumbles out of the ship; the other two are mysteriously missing - only their spacesuits remain. Uh-oh.
The surviving astronaut (Richard Wordsworth) is infected with something. The actor, Wordsworth, gets a lot of praise nowadays and he really creeped me out when I saw this as a kid (hey, I guess that's why they renamed it The Creeping Unknown). He begins to metamorphoses into something else and absorbs other living material. I didn't really get all this when I first watched it at a younger age; he first absorbs a small cactus plant by slamming it with his fist. Thereafter, his hand looks a bit on the monstrous side.
Brian Donlevy plays the scientist Quatermass in this and the 2nd film. Many fans feel he was miscast in the role; they feel Quatermass should have been more open to thoughtful introspection and even some regret over events in the films, despite his scientific bent (also, Quatermass is supposed to be British and Donlevy was not). But, Donlevy plays him with bulldog determination, never wavering and never showing much personality beyond a plain ornery attitude. In fact, the wife of the doomed astronaut finds Quatermass to be so annoying and obnoxious, she defiantly spirits her husband out of the hospital in an act of petulance, causing all the problems in the 2nd half of the film. Quatermass, as played by Donlevy, has no sympathy for her later, calling her an idiot, I think. Well, he may have had a point; her action was truly irresponsible.
The make-up FX involving the corpses were pretty effective for the fifties, still disturbing. Some of this reminded me of the later Island of Terror (1966). Things get very creepy in the last 15 minutes, when the audience doesn't really see what the astronaut has changed into; all we see is this trail of slime that indicates 'something' has passed through this way. The reveal at the very end, at Westminster Abbey, is naturally a letdown - think basically The Blob with tentacles. As always, our imaginations work better in cases like this. BoG's Score: 7 out of 10
A much later film, The Astronaut's Wife (1999), is pointed out by most familiar with this film as sort of a remake or rip-off of this old film. I also think of Species II (1998), in that the plot in that one also involved an astronaut coming back infected with something. There are also similarities to The Thing (1982), the John Carpenter version.