The novel "Blood on the Moon" upon which the movie is based was rejected by 17 publishers due to its violent content.
Originally filmed under the book's title 'Blood on the Moon', Atlantic, the studio behind the picture, decided to re-title the film "Cop" because they were concerned that people would think it was a slasher movie. The new title also suited Atlantic's market campaign, which tried to sell the film as an action/thriller similar to Dirty Harry.
Writer/producer/director James B. Harris and star/producer James Woods both worked for scale on this film in order to get it made.
Although the film garnered positive reviews, Atlantic, the studio behind the film, were encountering financial difficulties and could not publicize or advertise the film.
James Woods's character Lloyd Hopkins is in every scene in the film.