The uncertainty that exists in regard to the large mononuclear white blood cells is probably in large part due to the diverse forms they present with the usual stains, and the scarcity of each of these forms in normal blood. The several distinct types represented in this group do not have constant morphologic characteristics demonstrable by the usual stains that admit of a differential classification among them; and only occasionally is a case encountered in which one type stands out predominantly.1 Similarly the mononuclear wandering cells of the tissues, intimately associated with the mononuclear cells of the blood, are seen in many different forms, and although widely distributed throughout the body, each form is encountered alone in appreciable numbers only in rare pathologic conditions. The relations obtaining among these different mononuclear cells of the blood and tissues is, therefore, in some confusion and the status of each