In this study, organ and tissue samples were collected for routine histopathological examinations and also for the detection and definitive diagnosis of Marek's disease (MD) viral antigens by the Labeled StreptAvidin-Biotin Peroxidase (LSAB) method during post-mortem examinations of dead or sacrificed hens. The chickens were taken from various sites located in regions of intensive poultry production in Turkey. This disease is quite frequently encountered in Turkey and is still one of the major causes of economic losses in the poultry industry. The material of this study was composed of various organ samples collected from MD-suspected hens from various houses located within or around Istanbul, Adapazari, Bolu and Balikesir. Fifty hens were examined for the suspected disease. Sections prepared from collected organs or tissues were processed for both routine histopathological examinations and also for immunohistochemical evaluations under light microscope by the LSAB method. Lymphoid cell infiltrations of various sizes were observed, particularly in the liver and to a lesser extent in the kidney, spleen, proventricle and sciatic nerves. Various changes such as cell infiltrations through interfollicular areas and cyst formations in the follicular areas of the bursa of Fabricius were determined in the histopathological examinations of some of the samples of birds suspected of MD. In immunoperoxidase stained slides of birds suspected of MID, intracytoplasmic and less frequently intranuclear stainings were detected in a majority of antigen-positive cells.