<P>The fifth edition of this well-known series of lectures by distinguished Harvard hematologists is substantially revised to include discussions of the many important scientific developments in contemporary hematology. New insights into oncogenes, growth factors, membrane receptors, differentiation and morphogenesis, enzymology, molecular biology, and evolutionary biology appear in virtually every lecture. A lecture on general principles of malignant disease appears for the first time in recognition of the fact that in many hospitals and medical schools hematology departments now encompass oncology - an important topic that is not covered elsewhere in the medical curriculum.This new edition also adds many tables and figures relating to differential diagnosis that will be useful to practicing physicians and house officers. Examples include causes of splenomegaly, indications for splenectomy, differential diagnosis of normocytic anemia, macrocytic anemia, hypochromic anemia, thrombocytopenia, bone marrow failure syndromes, and many others. The bibliographies have been expanded, a new appendix containing laboratory methods has been added, and there is an unusually complete index.William S. Beck has taught the hematology courses at Harvard for many years. He is Professor of Medicine at the Harvard Medical School, Director of the Hematology Research Laboratory at the Massachusetts General Hospital, Professor at the Harvard-NUT Division of Health Sciences and Technology, and Tutor in Biochemical Sciences at Harvard College.</P>