Background: Newcastle disease virus (NDV) is a severe disease that afects domestic and wild birds. Controlled anti‑ biotics derived from probiotics have been examined as prospective solutions for preserving seroconversion in NDVvaccinated fowl. In this study, the secondary metabolite “telomycin” was extracted from Streptomyces coeruleorubidus (S. coeruleorubidus) isolated from Egypt’s cultivated soil. The structure of telomycin was determined by the elucidation of spectroscopic analysis, including nuclear magnetic resonance (NMR) and mass spectrometry (MS) spectra, and comparison with the literature. The antiviral activity of the secondary metabolite was tested by checking its efect on NDV hemagglutination activity (HA). Moreover, HA of NDV was tested after inoculation of NDV (control) and a combi‑ nation of telomycin and NDV in 10- days- specifc pathogen-free embryonated chicken eggs (SPF-ECE) daily candling. Histopathological examination was performed for chorioallantoic membranes and liver of SPF-ECE. Results: S. coeruleorubidus secondary metabolite “telomycin” showed complete hemagglutination inhibition (HI) activity of NDV strain (MN635617) with log106 infectivity titers (EID50/mL). The HA of NDV strain was 8 log2 and 9 log2 with 0.5% and 0.75% of chicken RBCs, respectively. Preserved structures of chorioallantoic-membranes (CAM) with dilated capillary networks were observed in the treated group inoculated with telomycin and NDV. Histological changes in SPF-ECE liver were examined after inoculation in ova to further characterize the telomycin efect. Telo‑ mycin and NDV mixture inoculated group showed preserved cytoarchitecture of hepatocytes with the presence of perivascular foci of lymphocytes. The group that was inoculated with telomycin alone showed normal histology of hepatic acini, central veins, and portal triads. Conclusion: S. coeruleorubidus telomycin is a promising bioactive agent that might be a biological weapon against a deadly chicken NDV that costs farmers a lot of money