Ovine Pulmonary Adenocarcinoma (OPA) is a natural lung cancer of the sheep caused by Jaagsiekte Sheep Retrovirus (JSRV). In the present study, 17 OPA affected lung samples were subjected to histopathological, histochemical and immunohistochemical studies. Grossly, lungs revealed diffuse areas of consolidation or tumor nodules and histologically lung sections showed proliferation of alveolar epithelial cells that were arranged in papillary or acinar pattern. The lung sections were further subjected to immunohistochemical staining for cytokeratin and Ki-67. The proliferative activity of tumor cells was determined by Ki67 and AgNor staining and the mean value of Ki-67 index was 9.53 + 1.88. The mean number of AgNORs for all the cases ranged from 6.5 to 11.9 with a mean + standard deviation values of 9.2 + 1.2. The mean values of Ki-67 index and AgNOR staining were increased in the sections of lung tumor than the normal lung sections indicating the increased proliferative activity of lung tumor epithelial cells.