Abstract
After a short summary of the few reported tumours of the male genital system in non-human primates, a malignant Leydig cell tumour is described in an adult male Tupaia belangeri. The tumour had metastasized in the omentum probably by haematogenous spread enabled by the peculiar perivascular growth pattern of the tumour cells. Its differential diagnosis versus seminomas and Sertoli cell tumours is discussed.