Bovine hyperimmune anti-Cryptosporidium colostrum immunoglobulin (BACI) decreases the intensity of Cryptosporidium parvum infection in vitro. We investigated the prophylactic effect of BACI in healthy adults challenged with C. parvum. After we established an oocyst dose that resulted in 100% infection in four volunteers (baseline group), 16 volunteers were randomized to receive (1) BACI prior to C. parvum challenge (BACI group) and a nonfat milk placebo 30 minutes later, (2) BACI prior to and 30 minutes after challenge (reinforced BACI group), or (3) nonfat milk placebo prior to and 30 minutes after challenge. Subjects received BACI (10 g) or nonfat milk placebo three times a day for a total of 5 days and were followed for clinical symptoms and oocyst excretion for 30 days. A trend toward less diarrhea (P = .08) was observed for subjects receiving BACI in comparison with occurrences in placebo recipients. Subjects receiving BACI or nonfat milk placebo had a 100-fold reduction in oocyst excretion as compared with excretion in the baseline group.