In March and April of 1993, Milwaukee, Wisconsin reported a very large increase in the number of diarrhea patients and shortages of over the counter drugs for diarrhea control at local pharmacies. Preliminary investigations conducted by State and City officials suggested that the drinking water may have been partially responsible for distributing Cryptosporidium around Milwaukee. In addition to turbidity data, a survey of diarrhea cases in local nursing homes implicated the Howard Water Treatment Plant. A team of investigators from the EPA's Drinking Water Research Division (DWRD) assisted with the investigation. This paper describes the approach the EPA team took in evaluating the effectiveness of the water treatment facility to remove particulates, including a discussion of the key operational data and a description of what was observed at the treatment facilities. Research including both extramural and in-house laboratory and pilot plant studies for Cryptosporidium removal is also discussed. A brief summary and discussion of this data is presented in this paper.