U.S. Army Iodine Water Purification Tablets were tested to determine their efficacy against Cryptosporidium parvum, a protozoan resistant to chemical disinfection. Purified oocysts in phosphate-buffered water were treated with varying concentrations of iodine or with iodine tablets as per U.S. Army protocol. Neonatal mouse pups were then each inoculated with 10,000 treated oocysts, and 1 week later scored as infected or uninfected. Using this methodology, iodine tablets were found to be inadequate against C. parvum because the Army doctrinal dose of 560 mg min/L, calculated as 16 mg of I2/L and 35 minutes of contact time, showed less than 1 log inactivation. A dose of 29 mg of I2/L at the same contact time was required to achieve a 2 log inactivation.