The purpose of this study was to investigate the effect of mannan oligosaccharide (MOS) mixed in the whole milk on the growth and health of calves. Forty Holstein calves (20 male, 20 female) were randomly assigned, within sex, to one of two groups at 5 d of age. Calves were fed fresh, warm whole milk (control) or fresh, warm whole milk supplemented with MOS (4 g of Bio-Mos per calf daily at a.m. feeding). Calves were fed 4 L of whole milk per day in two equal feedings for 6 weeks. Pelleted calf starter and fresh water were available ad libitum during the experiment. No hay was fed. Body weights (BW) were measured at the beginning of the experiment and then weekly till the end of the experiment. Height at withers, heart girth, withers to pins, and point of shoulder to pins were measured at the beginning and at the end of the experiment. Fecal fluidity scores and respiratory condition were monitored daily. Intake of whole milk was similar between the groups (P>0.10). Although there was no significant difference in starter intakes (P>0.10), calves in MOS treatment consumed 19.9% more calf starter than calves in control diet (499.2 vs. 416.5 g/d) for 26 to 46 d of age. BW gains for various age periods were unaffected by treatment (P>0.10). However, calves in MOS treatment gained 11.2% more BW than calves in control diet (627.4 vs. 564.2 g/d) from 26 to 46 d of age. Skeletal growth and feed efficiencies were similar for calves in both groups. No respiratory problems in calves in both groups occurred during the experiment. Fecal fluidity scores, percent calf days with scours and percent calf days treated for scours were unaffected by treatment (P>0.10). However, calf scours was a common problem especially during the first 3 weeks of the experiment. The reason for this may have been that calf scours had been a common problem at the location of the experiment for many years. This suggests that there must have been a significant enteric challenge for calves at this location. Even under these stressful conditions, MOS in whole milk did not elicit an improvement in enteric health in the circumstances of this location. Although there were no significant differences between the groups, the results suggest that starter intake and consequently BW gain from 26 to 46 d of age tended to be improved in calves in MOS group compared with calves in control group.