A Bay of Blood (Italian: Ecologia del delitto, lit. "Ecology of Crime", later retitled Reazione a catena [lit. "Chain Reaction"]) (also known as Carnage, Twitch of the Death Nerve and Blood Bath) is a 1971 Italian giallo slasher film directed by Mario Bava. Bava co-wrote the screenplay with Giuseppe Zaccariello, Filippo Ottoni, and Sergio Canevari, with story credit given to Dardano Sacchetti and Franco Barberi. The film stars Claudine Auger, Luigi Pistilli, Brigitte Skay, Nicoletta Elmi and Laura Betti. Carlo Rambaldi created the gruesome special make-up effects. The story details a string of mysterious murders that occur around the titular bay.

Widely considered Bava's most violent film, its emphasis on graphically bloody murder set pieces was hugely influential on the slasher film subgenre that would follow a decade later. In 2005, the magazine Total Film named A Bay of Blood one of the 50 greatest horror films of all time.