This paper discusses the preparation, protocol and results of the bacillus spore challenge test, which had never been carried out on a full scale operating water treatment facility. This test demonstrates the effectiveness of such a technique in evaluating membrane integrity of a full scale treatment facility, and the capability of a USF Memcor CMF plant to effectively reject particles smaller than pathogenic bacteria and protozoa of primary concern in drinking water applications. Based on spore dosing at high concentrations the plant achieved a