The right auricle is the primary site of leukotic changes in the bovine heart. This and other regions of the heart from 114 cattle with manifest leukosis, 44 cattle with suspected latent leukosis, 23 normal cattle, and 59 foetuses have been examined histologically. The propensity of bovine leukosis to involve the right auricle is obviously associated with the loose subepicardial connective tissue and its property of embryonal haematopoiesis. Since other mesenchymal tumours and metaplasia also involve the right auricle more often than the left, it is conceivable that the subepicardial tissue on the right side remains functionally as well as morphologically undifferentiated, and more readily responds to proliferative stimuli than does the subepicardial tissue of the left atrium, which early differentiates into fat tissue. Among the group of animals suspected of latent leukosis there were many with definite signs of leukosis in the subepicardial tissue of the right auricle. The early signs of leukosis were accumulation of lymphoid or blast-like cells, or both together, neural lymphoid infiltration, and stuffing of blood vessels with lymphocytes. The question whether the leukosis cells are formed in situ in the heart or represent metastasis from another site remains unsettled. The demonstration of a particular tissue type at the primary site of cardiac leukosis implies that further study might resolve this point and elucidate the early changes of leukosis.