Until recently the only biomedical use of erythrocytes was in transfusion medicine to restore a normal oxygen delivery. The development of a technology that permits one to open and reseal erythrocytes has dramatically changed this perspective. Currently, a number of teams have shown that engineered erythrocytes can behave as circulating bioreactors for the degradation of toxic metabolites or the inactivation of xenobiotics, as drug delivery systems, as carriers of antigens of vaccinal interest, and in many others biomedical applications. The technology of opening and resealing the erythrocytes has also been used successfully to investigate several basic aspects of erythrocyte metabolism, survival, pathology, etc. Thus, researchers in this field have an extraordinary opportunity to specifically modify the erythrocytes by the introduction of enzymes that generate new metabolic abilities, antibodies that inactivate single metabolic steps, or metabolites that can influence oxygen delivery and/or other cell properties. Furthermore, the pharmacokinetics of any drug can be potentially manipulated by using the erythrocytes as a delivery system. This book, The Use of Resealed Erythrocytes, is based on the fourth meeting of the "International Society for the Use of Resealed Erythrocytes as Carriers and Bioreactors" (I. S. U. R. E. ), held in Urbino, Italy, in 1991, and examines the most recent applications and developments of this technology.