The present paper describes cytochemical, membrane, functional, and mitotic characteristics of monoblasts, promonocytes, monocytes, and macrophages of the mouse, guinea pig, rat, and man. For all of these species the results show that after staining for nonspecific esterase, with alpha-naphthylbutyrate as substrate, and for lysozyme, mononuclear phagocytes can be distinguished from other cells, e.g., T and B lymphocytes. However, it must be kept in mind that immature and mature granulocytic cells are also lysozyme positive. The presence of Fc and C receptors is dependent on the maturity of the cells and the duration of incubation in vitro; with respect to the former, an in vivo population of immature mononuclear phagocytes may have a lower percentage of positive cells than is the case in a mature population, and with respect to the latter, the percentage of positive cells rises during incubation. Phagocytosis of opsonized bacteria and red cells is a reliable criterion for the distinction between mononuclear phagocytes and other cell types, e.g. lymphocytes and fibroblasts. In all of the species studied, the majority of both immature and mature mononuclear phagocytes ingested particles opsonized with IgG; the proportion of phagocytosis of red cells via C3 receptors is usually very small. Incorporation studies with [3H] thymidine have shown that immature mononuclear phagocytes (i.e., monoblasts and promonocytes) divided and that monocytes and macrophages do not. The small number of macrophages that incorporate [3H] thymidine are immature mononuclear phagocytes which have very recently arrived in the tissues from the bone marrow. Comparison of mononuclear phagocytes in different organs of various species has shown unequivocally that these cells belong to one cell line, called the mononuclear phagocyte system.