Disclosed is a single population of simulated leukocyte granules prepared from mammalian whole white blood cells, which is useful in calibrators for a hematology analyzer, a calibrator which includes the single population of simulated leukocyte granules. Disclosed is also a method of preparing the single population of simulated leukocyte granules. The method of preparing single population of simulated leukocyte granules includes the actions of: obtaining mammalian whole white blood cells; treating the mammalian whole white blood cells with a hemolytic agent and a leukocyte treating liquid which includes an isotonic saline solution. The isotonic saline solution may include a fixative, an oxidizer, and a carbohydrate compound; fixing the treated mammalian whole white blood cells with a fixative; and suspending the fixed cells in a preservation medium for long-term preservation of fixed cells.