This volume deals mainly with morphologic, ontogenic and metabolic aspects of phagocytosis and factors affecting RES function. The first section contains chapters on the morphology and surface properties of macrophages. The second section deals with the ontogeny and development of B and T lymphocytes as well as macrophages. The third section covers biochemical and physiologic parameters of mononuclear cells and of polymorphonuclear leukocytes, e.g., cyclic nucleotide and iron metabolism, chemotaxis and enzymatic activity, while the fourth is concerned with the phagocytic event. The fifth section relates to factors affecting RES function with discussions on surface receptors of B lymphocyte activation by antigens in regard to the effect of ligand concentration and induction time, lipopolysaccharide effects in vivo and in vitro as related to RES cells including a correlation of lipopolysaccharide structure with biological activity and, finally, the role of the RES in shock and trauma. Summaries of two workshops are also contained in this volume, the first workshop was concerned with the biochemistry and physiology of the RES; the second dealt with pharmacologically important aspects of RES metabolism and function. It is anticipated that the presentation of symposia and scientific papers and the workshop summaries will reflect the intense activity that is taking place in this multidisciplinary field. A better understanding of the Reticuloendothelial System is directly relevant to both conceptional and practical aspects of not only immunology, but other areas of medical and biological sciences in general.