The article presents the results of the study of a dog with severe acute leukemia. Acute leukemia in Caucasian Shepherd of 6 years of age has evolved during a month. The clinical picture was character-ized by progressive emaciation, loss of appetite, lethargy, fatigue at any physical exercises, pale mucous membranes, shortness of breath, tachy-cardia, submandibular lymph nodes enlargement. Blood serum revealed the increase in ALT and AST concentrations of 5.7 and 2.3 times respectively, alkaline phosphatase 3.1 fold, the decrease in the total protein 2 fold, the increase in creatinine con-centration in 18 %, and the reduction in calcium ions concentration was twice as much. At morpho-logical studying of blood there have been found poikilocytosis, anisocytosis, polychromatic erythro-cytes spherocytes and normocytic and erythrocytes with Heinz bodies. The number of red blood cells was 2.4 times reduced, the concentration of hemo-globin was 1.7 times, the hematocrit dropped to 2.0 times, the average concentration of hemoglobin in erythrocyte was above the reference value. In white blood the increase in leukocyte number by 3.3 times was observed, the absolute number of lym-phocytes was 2.2 times, the absolute number of monocytes was 1.5 times, and the absolute num-ber of granulocyte was 16.4 times. In the blood smears the accumulation of large blast cell was recorded. The dog owners’ withholding the pro-posed veterinary care contributed to the rapid de-velopment of multiple organ failure and death of the animal for a short time. For the differential diagno-sis of blast cells we can recommend using cytochemical Graham-Knoll method, which allows veterinarians to verify blast cells of granulocytic type by simple and reliable way.