The purpose of the study was to determine if immunization with a recombinant protein (rC7) of Cryptosporidium parvum would induce immune bovine colostrum that protected calves against cryptosporidiosis following oral challenge with C. parvum oocysts. Late gestation Holstein cows with low titers of antibody to the p23 antigen of C. parvum were immunized three times with 300 microg affinity purified rC7 C. parvum recombinant protein (immune cows), or left nonimmunized (control cows). Colostrum was obtained from each cow in both groups and partitioned into identical aliquots of pooled immune colostrum or pooled control colostrum. Twelve calves obtained at birth received either immune or control colostrum within the first 2 h, and again at 12 and 24 h of age. Each calf was challenged orally with 10(7) C. parvum oocysts at 12 h of age and monitored for signs of cryptosporidiosis. All six calves administered pooled control colostrum developed severe diarrhea (mean total fecal volume = 8447+/-5600 ml) and shed an average of 1.87+/-1.66 x 10(12) C. parvum oocysts. None of the six calves administered pooled immune colostrum developed diarrhea (mean total fecal volume = 740+/-750 ml, p < 0.05), and shed significantly fewer oocysts (3.05+/-2.26 x 10(9), p < 0.05). The absence of diarrhea and 2.79 log10 (99.8%) reduction in oocyst excretion indicates that immune bovine colostrum induced by immunization with C. parvum recombinant protein rC7 provided substantial protection against cryptosporidiosis in neonatal calves.