A case of acute myeloblastic leukaemia with unusual cytochemical reactivity in a dog is reported. Blood and bone marrow examination revealed predominantly highly undifferentiated leucocytes lacking peroxidase, sudan black B, and alpha-naphthyl acetate esterase activities. Most of the immature cells in blood and bone marrow smears showed strong alkaline phosphatase activity. Transmission electron microscopy of bone marrow cells revealed features of myeloid differentiation particularly secondary granules. These findings supported the diagnosis of acute myeloblastic leukaemia without differentiation (Ml) under the FAB system of classification of acute myeloid leukaemas in humans.