Technical Abstract: Two studies were conducted to evaluate the effectiveness of an allicin-based milk replacer additive (Enterocin-C) in decreasing the clinical effects of cryptosporidiosis in calves. For the first study, calves were orally-challenged with 1.5 x 10 6 Cryptosporidium parvum oocysts on day 1, with half of the calves (treated group) also receiving 2 packets of Enterocin-C in milk replacer (40 mg allicin/packet). Fecal consistency was scored twice daily. Calves developing diarrhea in the treated group received 2 additional packets at onset of diarrhea and continued to receive 1 packet daily until diarrhea ceased. Levels of diarrhea (total fecal scores; sum of days 4-21 P.I.) and average weight gains from the treated group did not differ significantly from those of the nontreated group (p=0.623 and 0.935 respectively). For the second experiment, the Cryptosporidium challenge was halved, and the dose of Enterocin-C intensified. Treated calves received 2 Enterocin-C gelatin capsules (80 mg allicin/capsule) commencing on day 1 for 7 consecutive days, regardless of fecal scores. Calves were challenged with Cryptosporidium on day 2. These results again failed to demonstrate any significant changes in levels of diarrhea or weight gains in challenged calves treated with this dose of milk additive as compared with the challenged- nontreated controls.