Objectives: Respiratory complexes are serious threats to poultry industry causing a heavy financial burden on the owners of the industry and governments. Avian influenza virus (AIV) and Newcastle disease virus (NDV) are the two most important respiratory agents in poultry production. Decreased efficacy of production and increased mortality rate are observed in affected flocks. Serological assays are useful tools to evaluate immune status of birds. Hemagglutination inhibition (HI) test is a serologic test which is commonly used in diagnostic laboratories. Material and Methods: In order to determine NDV and AIV (H9N2 subtype) antibody titers in broiler flocks of West Azarbayjan, blood samples were collected from 64 flocks of West Azarbayjanavicultures and subjected to HI. The results were recorded and analyzed using version 18 of SPSS statistical software. Results: Out of a total number of 1884 birds tested for AIV, 7.27% (137 birds) were sero-negative showing antibody titers of 0-1, 41.89% (789 birds) had an antibody titer range of 2-5 which may be due to previous vaccination, and 50.84% (958 birds) had antibody titers of 6-12 which are considered as possible field challenge. Identically, serum antibody titers for NDV were as the following: 250 (12.37%) out of a total of 2020 birds examined for NDV showed negative antibody titers (0-3), 948 birds (46.93%) had serum antibody titers of 4-7 which may be due to previous vaccination, and 822 birds (40.69%) were in a range of 8-12 (possibility of field challenge). There were some differences in antibody titer status of birds between different seasons of year. Conclusion: High HI antibodies of AIV and NDV in serumof birds highlights important role of these infections in respiratory complexes of broilers in this area. Biosecurity measures, vaccination and monitoring are effective tools to prevent introduction of such infections and decrease financial losses due to these infections.

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