This book is an edited verbatim account of the proceedings of the Second London Workshop on Red Cell Filtration and Deformability, organised jointly by the Royal Society of Medicine and the Groupe de Travail sur la Filtration Erythrocytaire. The first such London Workshop was held on the occasion of the Second European Conference on Clinical Haemorheology in 1981 and its con clusions were published in an abbreviated form. The interest in blood filtration as a technique for assessing red cell deformability has continued to increase and there are now several hundred centres around the world using some form of filtration. It was, therefore, felt that the informal detailed discussions held by a small invited group of experts from many countries would be of more general interest and therefore worth publishing. This has been made possible by the continued interest and generous support of Hoechst. Spontaneous open discussion rather than formal presentations has been the hallmark of these workshops, with precirculation of all scientific data to the participants. We have attempted to preserve this flavour in editing the transcript of the proceedings. This, together with our anxiety for rapid publication, may have resulted in a less than finely polished text which may contain some in accuracies. The topics discussed were loosely structured into eight sessions, outlined in the accompanying programme.