Recent outbreaks of disease associated with water contaminated by the parasite Cryptosporidium have heightened public awareness of the need for pure drinking water. Since most common methods of microorganism control show little effect against Cryptosporidium, bottled water companies are actively evaluating alternative systems to assure purity for the consumer. In this article, Dr Norbert Roessler, Cuno Inc., USA describes one such system which employs a graded-densify, polypropylene filter cartridge that acts as an effective final barrier against cyst contamination. The performance of this filter has been confirmed by the National Sanitation Foundation according to Standard 53 for cyst removal which uses a dust surrogate challenge. Comparable results were obtained when the filter was tested by a third party laboratory using an active, live cyst challenge.