Fourteen groups of young commercial chickens were immunized once with a live NDV vaccine using different vaccine doses and routes of vaccination in five experiments. Three to six weeks later, small groups were selected from each flock. Sera were tested by the haemagglutination-inhibition test and a monoclonal antibody blocking ELISA, and the birds were challenged with a virulent NDV strain. Degree of protection was dose-dependent in those groups where the vaccine was administered orally at 3 weeks of age. Aerosol and eye drop vaccinations performed in day-old chicks provided full protection at 5 or 6 weeks of age. There was a good agreement between the two serological methods and positive results in any of the tests were reliable forecasts of protection.