Summary:Twelve foals with difficulty in respiration that have died during spring-summer months between 2003-2006 years were necropsied. On necropsy, pneumonia was detected in all of the foals. In 5 animals, caseo-necrotic nodules were seen in the lungs. In histopathological examination, purulent bronchopneumonia was noticed. In immunohistochemical staining, Rhodococus equi antigens were determined in the cytoplasm of macrophages and occasionally in neutrophils in lungs and mediastinal lymph nodes by immunoperoxidase technique. No lesions were detected in the intestines confirming that this form of infection is rare compared to the lung form. In the present study, R. equi was microbiologically isolated only in two cases. In conclusion, R. equi infection was diagnosed in all of the twelve cases, and estimated to be the reason of pneumonia in the foals. It was also shown that immunoperoxidase technique can be successfully used for R. equi infection in the field.