Photopolymer printing plates having, on top of a photosensitive layer (a) attached to a base, a thin layer (b) of a suspension of finely crystalline conjugated diacetylene compounds in a binder, which compounds change color when photopolymerized. Such printing plates make possible the production of positive copies of a positive transparency by producing in layer (b) an image with areas which do not transmit light by exposure to light having a wavelength of less than 320 m Mu and optional fixing, and initiating photopolymerization in layer (a) by exposing it through the said image to light having a wavelength of from 320 to 450 m Mu and then washing out the unexposed areas of layer (a).