Technical Abstract: The present study is being conducted to determine the risk associated with using live recombinant NDV(rNDV) vaccines in the field. The goals of this study are to 1) determine the risk of rNDV vaccines, containing an attenuated fusion (F) protein cleavage site, to revert back to a virulent virus phenotype; and 2) to assess the ability of rNDV vaccines to infect and spread in non-target species. Infection of 14-day-old embryonating chicken eggs (ECEs) with wild type NDV strains LaSota and Australia or with rNDV strains LaSota (rLaSota), or ZJ1 (rZJ1*Lento) (900 eggs/virus), resulted in the death of 65 (7.2%), 10 (1.1%), 23 (2.5%), and 6 (0.6%) embryos, respectively within 72 h pi. Sequencing of the F gene cleavage site revealed that, under our experimental conditions, all recombinant viruses are stable and did not revert to a virulent phenotype. Experiments to assess the ability of recombinant NDV vaccines to infect and spread in pigeons have shown that rLaSota and rLaSota/AIV-H5 are able to infect and spread in this species. All virus-inoculated birds shed the virus in oral and/or cloacal secretions and at least one out of four contact birds shed the virus for two or more days. These findings suggest that our system is suitable to assess the risk associated with rNDV vaccines.