The histological and immunohistochemical characteristics of a malignant Sertoli cell tumour in a 17-year-old Amur tiger. (Panthera tigris altaica) are described. Histological examination of the primary lesion in the right testis and metastatic lesions throughout the internal organs showed a variable cellular pattern with an admixture of tubular structures divided by fine stroma filled with fusiform to stellate cells, and sheets of polygonal cells with abundant vacuolated cytoplasm. Immunohistochemical techniques demonstrated strong positive staining for neuron-specific enolase and variable positive staining for vimentin in neoplastic cells, supporting a diagnosis of a tumour of Sertoli cell origin. (C) 2001 Harcourt Publishers Ltd.