The film was produced by Anthony Hinds, directed by Val Guest, and stars Brian Donlevy as the titular Professor Bernard Quatermass and Richard Wordsworth as the tormented Carroon.

After Carroon escapes from custody Quatermass and Inspector Lomax (Jack Warner) of Scotland Yard have just hours to track him/it down and prevent a catastrophe.

Meanwhile, Scotland Yard Inspector Lomax undertakes investigation of Reichenheim and Green's disappearance and, having fingerprinted Carroon as a suspect, alerts Quatermass that the prints are nothing human.

Inspector Lomax initiates a manhunt for Carroon, who goes to a nearby pharmacy and kills its chemist, using his cactus-thorn-riddled hand and arm as a cudgel and leaving a twisted, empty man-husk to be found by the police.

On a police tip from a vagrant, Lomax and his men track the Carroon mutation to Westminster Abbey, where it crawled high up on a metalwork scaffolding.

Written by Nigel Kneale, it was an enormous success with critics and audiences alike, later described by film historian Robert Simpson as "event television, emptying the streets and pubs".

[7] Nigel Kneale also saw the potential for a film adaptation and, at his urging, the BBC touted the scripts around a number of producers, including the Boulting Brothers and Frank Launder and Sidney Gilliat.

[11] Hammer's offer met some resistance within the BBC, with one executive expressing reservations that The Quatermass Experiment was not suitable material for the company, but the rights were nevertheless sold for an advance of £500.

[13] This became a matter of some resentment on Kneale's part, and when his BBC contract came up for renewal he demanded and secured control over any future film rights for his work.

[17] The first draft of the screenplay was written by Richard Landau, an American provided by Lippert who had worked on six previous Lippert-Hammer productions, including Spaceways (1953), one of the company's first forays into science fiction.

[27] Having fallen foul of the censors with some of their earlier films, Hammer had an informal agreement to submit scripts in advance of shooting for comment by the BBFC.

[6] Irish-American actor Brian Donlevy was brought in by Robert L. Lippert to play the title role of Quatermass to provide an interest for American audiences.

[38] By this stage in his career, Donlevy was suffering from alcoholism; it was some weeks into the shoot before Guest became aware that the flask of coffee he always carried on set was laced with brandy.

[43] Shortly after finishing The Quatermass Xperiment, he made his first appearance on television in the role he is most associated with: the title character in Dixon of Dock Green (1955–76).

[56] Guest had little interest in science fiction and was unenthusiastic about directing the film; he reluctantly took copies of Nigel Kneale's television scripts with him on holiday in Tangiers and only began reading them after being teased for his "ethereal" attitude by his wife, Yolande Donlan.

[66] The shots of the emergency services rushing to the rocket crash site at the beginning of the movie were filmed in the village of Bray, Berkshire, where Hammer's studios were located.

[69] A second unit, under cameraman Len Harris, conducted additional location shooting around London for the montage scenes of the police search for Carroon.

[30] Art director James Elder Wills, in his final film for Hammer, made great use of the existing architecture of Down Place to enhance the effectiveness of his sets.

[71] The work of makeup artist Phil Leakey in transforming Richard Wordsworth's Carroon into the mutating creature was a key contribution to the effectiveness of the film.

[92] Whereas most other studios were nervous of this new certificate, Hammer, who had noticed the success of the similarly 'X'-rated Les Diaboliques (1955),[93] chose to exploit it by dropping the "E" from "Experiment" in the title of the film.

[99] In some parts of the UK, the Watch Committees of local councils demanded certain scenes, mainly close-up shots of Carroon's victims, be removed before allowing the film to be exhibited in their jurisdictions.

[75] In the United States, Robert L. Lippert attempted to interest Columbia Pictures in distributing the film but they felt it would be competition for their own production, It Came From Beneath The Sea, which was on release at the time.

[96] The Creeping Unknown was packaged as the bottom half of a double bill with a Gothic horror movie called The Black Sleep, starring Basil Rathbone, Lon Chaney Jr. and Bela Lugosi.

[101] According to a report in Variety, published on 6 November, a nine-year-old boy died of a ruptured artery at a cinema in Oak Park, Illinois during a showing of this double bill.

[102] The Times newspaper critic gave the film a generally favourable assessment: "Mr. Val Guest, the director, certainly knows his business when it comes to providing the more horrid brand of thrills...

[104] Another unimpressed critic was François Truffaut, who wrote in Cahiers du cinéma that "this one is very, very bad, far from the small pleasure we get, for example, from the innocent science fiction films signed by the American Jack Arnold...

[106] Upon its release in the United States Variety noted "Val Guest's direction brings out the maxiumum suspense factors" and found the movie to be "a competently made drama .

[107] According to Bruce Hallenbeck, many US critics found Brian Donlevy's gruff Quatermass a breath of fresh air from the earnest hero scientists of American science fiction films, such as Gene Barry's character in War of the Worlds.

(1982) found that "the buildup is slightly too long and too careful"[112] but also said that "it's an intelligent, taut and well-directed thriller; it showcases Nigel Kneale's ideas well; it's scary and exciting.

[121] Nevertheless, the film went ahead, as X the Unknown (1956), again capitalising on the 'X' Certificate in its title and featuring a newly created scientist character, very much in the Quatermass mould, played by Dean Jagger.