Sixty female Holstein calves were used to study the effect of mannan oligosaccharides (MOS) on performance, health, bacteria fecal counts and Cryptosporidium presence in feces of calves following an enhanced-growth feeding program. Calves were divided in two groups: supplementation of 4g/d of MOS on milk replacer (MR-M) or non-supplemented milk replacer (MR-C). After 1wk of adaptation to milk replacer (MR) at 180g/kg dilution, calves were fed: 4l/d of MR from 1 to 7d, 6l/d from 8 to 14d, 7l/d from 15 to 21d, 6l/d from 22 to 28d, and 3l/d once daily in the afternoon meal from 29 to 34d. Calves were weaned at 35d of study, and were offered water and starter ad libitum until 41d of study. Starter and MR intake were recorded daily. Calves were weighed weekly, and blood samples were obtained at 1, 3, 5 and 6wk of study to determine haptoglobin serum concentrations. Moreover, total fecal counts of Clostridium perfringens and Escherichia coli, and for presence of Salmonella spp. and Cryptosporidium spp. were determined from fecal samples obtained at 1, 2, and 3wk of study. There were no differences in body weight, but starter intake tended to be greater (P=0.09) during the pre-weaning period (0.34 versus 0.28±0.023kg/d) and greater (P