Loreta Tovar and María Luisa Tovar have a lesbian sex scene together in this film, despite being sisters in real life.
This film doesn't follow the classic vampire lore, as here the vampires resemble werewolves or "were-vampires" more closely: they seem to be normal humans by day, moving freely under the sunlight and casting reflections in the mirror, and only turn into vampires under a full moon.
Filmed in a double version: one with nudity and sex scenes for international sales, and a tamer one, replacing the above with fully clothed scenes, for domestic consumption under the heavy Francisco Franco era censorship.