The antimicrobial properties of magnesium monoperoxyphthalate hexahydrate (MMPP), which is a stable, water-soluble solid peroxyacid, have been investigated. A 2% (w/w) solution of MMPP rapidly kills yeasts and vegetative bacteria and slowly inactivates bacterial endospores at 22 degrees C. Biocidal activity, which is greater under mildly acidic conditions, is retained in the presence of organic contamination or hard water. Sporicidal action is greatly increased by a moderate rise in temperature or by using MMPP in combination with propan-2-ol. Such mixtures appear to be suitable for use as liquid chemical sterilants.