In April of 1993, Milwaukee, Wisconsin, became the major focus of the drinking water industry. The focus of attention centered on the large increase in reported cases of diarrheal patients throughout the city. Although other organisms may have been involved, the major increase in diarrhea was determined to be caused by the organism Cryptosporidium. Investigations suggested that drinking water may have been partially responsible for distributing the organism in Milwaukee and the surrounding area. Pilot plant and laboratory studies using various filtration techniques have been used to evaluate the ability of the filtration systems to remove specific microorganisms. In early studies, Cryptosporidium parvum oocysts were used in jar test studies. One of the conventional coagulation/filtration pilot plant systems was challenged with a source water containing 100,000 oocysts/liter. The pilot slow sand filter was also challenged with water containing 100,000 oocysts/liter. Slow sand filter tests were conducted with Cryptosporidium at Pennsylvania State University. The preliminary tests have indicated that most filtration systems are effective in removing 3 logs of oocysts under ideal operating conditions. Several surveys in the literature have indicated that oocysts are passing through some filter plants. The passage of oocysts may indicate that fresh (or untreated) oocysts may be more difficult to remove than those that are pretreated by some manner.