Method and apparatus for comminuting cellulosic materials such as paper and destructible constituents of trash and garbage wherein the material is supported on a flat wire screen presenting sharp, upwardly directed edges and wherein the material is subjected to the force of jets under a high enough pressure to impact the material at at least 100 psi. The disclosure provides for varying the force of jets to accommodate differing qualities of material and to selectively comminute only that which is desired to be introduced into a slurry, the force being insufficient to comminute materials to be rejected.