When compared with normal milk, bovine colostrum contains a large amount of uridine 5'-monophosphate (UMP) and its derivatives. In the present study, we carried out 2 experiments to determine the effects of dietary UMP (2 g/d) on the immune status of newborn calves. In Exp. 1, newborn Holstein bull calves were fed milk replacer alone (control group) or milk replacer supplemented with UMP (UMP group) from d 4 to 10 after birth. The increase in interferon-gamma concentration by peripheral blood mononuclear cells (PBMC) on d 24 tended to be greater in the UMP group than in the control group (P = 0.06). The IgA concentration of the ileal mucosa was greater in the UMP group than in the control group (P < 0.05), although there was no difference between groups in the jejunal mucosa. In Exp. 2, newborn Holstein bull calves were fed milk replacer alone (control group) or milk replacer supplemented with UMP (UMP group) from d 4 to 56 after birth. The proliferation of PBMC was greater in the UMP group than in the control group on d 14, 28, and 42 (P < 0.01). The increase in interferon-gamma concentration by PBMC was greater in the UMP group than in the control group on d 28 and 42 (P < 0.05). From these results, we concluded that dietary UMP affected the immune responses of newborn calves.