It is the purpose of this paper to present two cases of malignant endocarditis associated withStreptococcus uiridanssepticemia, showing some unusual cells termed macrophages in the peripheral blood. The morphologic and functional characteristics of these cells corresponded closely with cells produced experimentally in rabbits by one of us.1It is because of this similarity that we hope to offer some suggestions which may help to clarify the nature and relationship of these and other mononuclear cells found in the blood. The cells which we term macrophages (Metchinkoff, H. M. Evans) because one of their striking characteristics is a marked power of phagocytosis, have been synonomously termed pyrrhol cells (Goldman), adventitia cells (Marchand), histogenous macrophages (F. A. Evans), histiocytes (Aschoff, Kiyono), resting wandering or polyblast cells (Maximow), rhagiocrine cells (Renaut), endothelial leukocytes (Mallory), and clasmatocytes (Ranvier), although some of the above terms included in their scope additional mononuclear cells to