The structure of buffy coat from normal and leukemia rats was studied. Normal buffy coat in a microhematocrit tube showed a distinct stratified structure consisting of leucocytes, platelets, and leucocyte debris from the bottom to the top. Two distinct layers were further identified in the leucocyte layer in many rats. They were composed of polymorphonuclear leucocytes (bottom) and lymphocytes (top). From the prominent change in the thickness of each layer during 7,12‐dimethylbenz(a)anthracene leukemogenesis, this technique was shown to be useful for screening, follow‐up, and classification of the leukemia in this experimental sys