Eighty day-old broiler chicks vaccinated against Newcastle disease (ND) were randomly divided into two groups. Every morning for 21 days, the treated group received 1 L of 0.1 % D-mannose solution as a replacement for drinking water whereas the control group was given the same amount of tap water. Blood samples were collected on days 37 and 52 and analyzed for total and absolute white blood cell (WBC) counts. Antibody titers to ND were measured by subjecting the serum samples to hemagglutination-inhibition (HI) test. The D-mannosetreated group developed significantly higher (P<.05) levels of HI antibodies compared to the control on both days of blood collection. A considerable increase in the total WBC count of the D-mannose-treated group was likewise noted, which could be attributed to the higher absolute counts of heterophils, eosinophils, monocytes and lymphocytes. Results of this stud showed that oral supplementation with 0.1 % D-mannose offers considerable potential to enhance the immune response of broiler chickens vaccinated against ND.