Cryptosporidium parvum is a major cause of diarrheal disease in humans and has been identified in 78 other species of mammals. The oocyst stage, excreted in feces of infected humans and animals, has been responsible for recent waterborne outbreaks of human cryptosporidiosis. High temperature and long exposure time have been shown to render oocysts (suspended in water) noninfectious, but for practical purposes, it is important to know if high-temperature--short-time conditions (71.7 degrees C for 15 s) used in commercial pasteurization are sufficient to destroy infectivity of oocysts. In this study, oocysts were suspended in either water or whole milk and heated to 71.7 degrees C for 15, 10, or 5 s in a laboratory-scale pasteurizer. Pasteurized and nonpasteurized (control) oocysts were then tested for the ability to infect infant mice. No mice (0 of 177) given 10(5) oocysts pasteurized for 15, 10, or 5 s in either water or milk were found to be infected with C. parvum on the basis of histologic examination of the terminal ileum. In contrast, all (80 of 80) control mice given nonpasteurized oocysts were heavily infected. These data indicate that high-temperature--short-time pasteurization is sufficient to destroy the infectivity of C. parvum oocysts in water and milk.