The present study was conducted to diagnose viral diseases viz. Chicken Infectious Anaemia(CIA), Infectious Bursal Disease (IBD) and Avian Reovirus (ARV)infection causing immunosuppression in commercial poultry flocks. Tissue samples were collected from affected birds showing lesions in lymphoid organs. The formalin-fixed, paraffin embedded lymphoid tissues were processed for assessing histopathological changes and localization of viral antigens associated with immunosuppression in chickens under field conditions. The histopathological scores in lymphoid organs were correlated with localization of viral antigens in lymphoid organs using Pearson correlation coefficient. The results indicated high prevalence (23%) of mixedinfection of CIA, IBD and ARV. Among various viral diseases, highest incidence of CIA (18%) was found to be statistical significant (÷2 =9.23; P<0.05). A significant positive correlation was observed between histopathological score and immonohistochemical localization of viral antigens in thymus (0.60; P<0.05), spleen (0.63; P<0.05), bursa (0.51; P<0.05) and caecal tonsils (0.80; P<0.05). In conclusion, Chicken Anaemia Virus (CAV) causes mixed infection with other immunosuppressive viruses under field conditionsand imunohistochemical localization of these viral antigens in the formalin-fixed, paraffin-embedded tissue sections can be used effectively for differential diagnosis of mixed infection.