The International Life Sciences Institute (ILSI) was estab lished in 1978 to stimulate and support scientific research and educational programs related to nutrition, toxicology, and food safety, and to encourage cooperation in these programs among scientists in universities, industry, and government agencies to assist in the resolution of health and safety issues. To supplement and enhance these efforts, ILSI has made a major commitment to supporting programs to harmon ize toxicologic testing, to advance a more uniform inter pretation of bioassay results worldwide, to promote a common understanding of lesion classifications, and to encourage wide discussion of these topics among scien tists. The Monographs on the Pathology of Laboratory Ani mals are designed to facilitate communication among those involved in the safety testing of foods, drugs, and chemicals. The complete set will cover all organ systems and is intended for use by pathologists, toxicologists, and others concerned with evaluating toxicity and carcinogen icity studies. The international nature of the project - as reflected in the composition of the editorial board and the diversity of the authors and editors - strengthens our ex pectations that understanding and cooperation will be im proved worldwide through the series. Alex Malaspina President International Life Sciences Institute Preface This book, on the hemopoietic system, is the eighth volume of a set prepared under the sponsorship of the International Life Sciences In stitute (ILSI).