TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preparation (Claimed): Preparation of rifaximin in amorphous form involves: a) pre-arranging the rifaximin in the form of powder; and b) subjecting the powder to mechanical impacts with at least one milling body so that an integral energy density equal to at least 50 kJ/g is transferred, where the integral energy density is so as to make the rifaximin assuming an amorphous state having an X-ray diffraction pattern as given in the specification. In the step (a) rifaximin is mixed with amorphous silica. The step of subjecting the powder to mechanical impacts involves a step of depositing the powder within a reactor of high energy ball milling type, where the reactor includes milling bodies inside it. The reactor is set in motion at a speed of 900-2000 revolutions per minute and for 60-180 minutes. The method involves spray drying a rifaximin solution in an atomizer, by using a nozzle flow rate of 0.5-20 ml/second and a gas at, at least 50 degrees C. The milling bodies are manufactured in a material comprising metals or their alloys or oxides. Preferred Components: The solvent in the rifaximin solution is preferably methylene chloride, but it can also be ethanol, methanol, ethyl acetate, chloroform, benzene and/or dimethylformamide. The integral energy density is 90-100 kJ/g. The rifaximin concentration in the solution is 1-30 wt./vol.%. The gas is air or nitrogen. Preferred Compounds: The rifaximin in amorphous form has X-ray diffraction pattern as given in figure 8 or in figure 9 or in figure 14 of the specification. ACTIVITY - Antidiarrheic. No biological data given.    MECHANISM OF ACTION - None given.    ADMINISTRATION - Administration is orally. No dosage details given.    EXAMPLE - Rifaximin (0.7 g) in the form of powder was placed into a jar. The jar was closed and made integral to an apparatus of shaker type, actuated at 900 revolutions/minute, equal to a frequency of impacts of 60 impacts/second. Under optimum conditions the obtained energy transfer was in the order of 90-100 mJ per impact. The time length of the mechano-chemical action was equal to 15 minutes. The recovered material had amorphous structure.