Despite Kneale's reservations about the casting, "The Quatermass Xperiment" was the highest-grossing film Hammer had made up to that point in its history, and has since been described by one academic as "the key British science fiction film of the 1950s". Hammer was keen to make an immediate follow-up, and wanted to use Quatermass in its 1956 film "X the Unknown"; however, Kneale refused Hammer the rights, and the company created its own substitute character, Doctor Adam Royston. Hammer did release an adaptation of "Quatermass II" in 1957, called "Quatermass 2" and this time with Kneale's involvement in the script. To the writer's displeasure, Donlevy returned as Quatermass.