Mononuclear phagocytes, which include macrophages, monocytes and their precursor cells, are the most important cells in the host defence against micro-organisms and tumor cells. During the last twenty-five years research on the biology of mononuclear phagocytes has increased tremendously. This motivated Professor R. van Furth to organize five international conferences on this subject in Leiden, the Netherlands. The edited proceedings of these meethings were published: in 1970 Mononuclear Phagocytes; in 1975 Mononuclear Phagocytes in Immunity, Infections and Pathology; in 1980 Mononuclear Phagocytes — Functional Aspects; and in 1985 Mononuclear Phagocytes — Characteristics, Physiology and Function. Reviews of these volumes, published in international journals, praised them as the most up-to-date state of the art publications. The publication of 1991 includes 88 chapters written by more than 200 authors.