In a method of and apparatus for rendering subject matter which is printed on one surface of a paper sheet (11a) unintelligible, an alpha-numeric interference pattern (Fig. 7) is printed over substantially the entire printing area of the surface of the sheet (11a) carrying the printed subject matter, the density of the interference pattern (Fig. 7) being sufficient to render the printed subject matter unintelligible. One particular use of the method is in the overprinting of computer print-out paper having printed information on one surface, so that the paper may be reused by printing on its opposite surface without permitting access to the information on the first-printed surface. A machine (10) is disclosed for this application of the method.