The antiviral and immunomodulatory activities of Xian Ren Zhang (Haworth or Opuntia monacantha) polysaccharides (OMP) were investigated. In vitro, neonatal hamster cell cultures cells were exposed to New Castle disease virus (NDV) and different concentrations of OMP (2.5-31.25 mg/ml) in 3 protocols: pretreatment (OMP first then NDV), virucide (OMP and NDV together) and therapy (NDV first then OMP). All protocols had a positive control (cells and virus) and blank control (cells only). Cell viability was represented by optical density (A₅₇₀) and half maximal inhibitory concentration (IC₅₀) values. The A₅₇₀ values of the 10, 20 and 31.25 mg/ml OMP groups were significantly greater than control groups. The IC₅₀ value in the pretreatment group was significantly less than the virucide or therapy groups. For the in vivo study, 300 chickens, 14 days of age, were randomly divided into 5 groups. Chickens in 4 groups were vaccinated with the NDV IV strain vaccine and the 5th group was an unvaccinated control. Following vaccination, chickens in 3 groups received 0.04, 0.08 or 1.6 g/kg OMP orally daily for 5 days. On days 7, 14, 21 and 28 after vaccination, serum anti-NDV antibody titers and intestinal mucosal secretory immunoglobulin A (sIgA) concentrations were determined. Antibody titers and sIgA concentrations in the group receiving 0.16 g/kg OMP were significantly higher than the control. These data suggested that OMP has anti-NDV activity in vitro and immune-enhancing activity in vivo and may be a potential new drug for the prevention and treatment of Newcastle disease in poultry.