The story is that while filming Flesh for Frankenstein Paul Morrissey and crew discovered they were quite ahead of schedule and under budget, so they decided to make a second movie. Released the following year, Blood for Dracula, which shares Frankenstein's critique of sexual promiscuity, was partially improvised and for some reason has a cameo from Roman Polanski. It's also a much more entertaining movie no matter what the Rotten Tomato ratings suggest, despite Joe Dallesandro's character being much more overbearing and hard to handle.