Acute myeloblastic leukaemia in a cow.

A myeloblastic leukaemia was found in a 6-year-old Holstein cow. Although macroscopical lesions were not found in parenchymal organs, the animal had many neoplastic cells in the peripheral blood, bone marrow, lymph nodes, and spleen. By light microscopy most neoplastic cells appeared to be agranular blast cells, but some cells contained acidophilic granules and stained positively for naphthol AS-D chloroacetate esterase and lysozyme. Ultrastructural examination revealed that many of the blast cells had a few peroxidase-positive granules and corresponded to early promyelocytes.