Ovine or bovine colostrums with different antibody titers were tested for their ability to prevent cryptosporidiosis in five groups of neonatal lambs experimentally infected with 10(6) Cryptosporidium parvum oocysts 2 days after birth (Day 0). In a control group (Group 1), six lambs were deprived of ewe colostrum and exclusively fed with milk replacer. Two groups of six lambs were allowed to suckle their non-hyperimmunized (Group 2) or hyperimmunized (Group 3) dams throughout the experiment. Two groups of seven lambs were separated from their dams at birth before suckling and fed with non-hyperimmune (Group 4) or hyperimmune (Group 5) bovine colostrum; for 7 days they received 50 ml of colostrum completed by milk replacer twice a day, then they were fed with milk replacer exclusively. Control lambs became infected and developed clinical cryptosporidiosis with diarrhea on Days 4-9 post inoculation, oocyst shedding and mortality (2/6). In all the treated groups, the colostrum prevented mortality and clinical cryptosporidiosis. The mortality (5/7) observed in Group 5 was not due to cryptosporidiosis but anemia. In treated groups, specific antibodies were detected on Day 0 after 2 days of colostrum intake and varied little in time for IgM and IgG in spite of the parasite development, whereas they appeared later in the control group, on Day 4 for IgM, Day 11 for IgA and Day 14 for IgG. In all groups, the response which was the most consistent was the IgA response which evolved from Days 11 to 18 in association with the decline of oocyst shedding. Our results show that whatever the serum antibody titers were, the specific C. parvum circulating antibodies have no influence on the control of cryptosporidiosis. The prophylaxis or the treatment of cryptosporidiosis require high titers of specific C. parvum antibodies in the gut lumen during a sufficiently long period.