This volume is a collection of the presentations given at the sixth international research conference, Blood Cells as a Tissue, at The Lankenau Hospital, October 30 and 31, 1969. The conference was supported in part by the following pharmaceutical industries: Abbott Laboratories, North Chicago, Illinois; Baxter Laboratories, Inc. ,. Morton Grove, Illinois; Bristol Laboratories, Syracuse, New York; Burroughs Wellcome and Company, Tuckahoe, New York; Eli Lilly and Company, Indianapolis, Indiana; Geigy Pharmaceuticals, Ardsley, New York; Merck Sharp & Dohme Post Graduate Program, West Point, Penn­ sylvania; Riker Laboratory, Northridge, California; Schering Corporation, Bloomfield, New Jersey; Squibb Research Institute, New Brunswick, New Jersey; and Smith Kline and French Labora­ tories, Philadelphia, Pennsylvania. This valuable support is gratefully acknowledged. The Conference Committee is especially indebted to the four chairmen, Dr. Allan J. Erslev, Dr. William J. Williams, Dr. R. Philip Custer and Dr. Sol Sherry, for their advice and guidance in the formulation of the program. Weare also grateful to the speakers for their splendid cooperation, and to all participants for their stimulating interest. The continuing support of the members of the Board of Trustees, of the Hospital Administration, and the Women's Volunteer Service played a major role in the success of the conference. We wish to express our sincere gratitude to the many members of the Hospital staff who contributed so willingly of their time to make the conference possible. A special vote of thanks goes to Mr. Ralph Hollerorth, Mrs. Carolyn Hyatt and Mrs.