PURPOSE: To easily and stably prepare a rat bone marrow specimen suitable for an automatic blood corpuscle classifying apparatus without requiring special technique.

CONSTITUTION: A nucleated blood corpuscle (leucocyte) and platelet are removed from the blood collected from a rat and an unnecessary component such as serum is removed from the blood after removal and the finally obtained blood of which the greater part is composed of an erythrocyte is mixed with the bone marrow of the rat to be centrifugally applied. By this method, a specimen composed of a single layer wherein a large number of erythrocytes are present around a cell and the dispersion of a blood corpuscle is uniform can be prepared easily and stably.