The use of aqueous two-phase systems for the partitioning of macromolecules, organelles and cells was originally developed by Per-Ake Albertsson in Sweden in the mid-fifties [1-3]. These systems were initially applied to separations of plant organelles and viruses but their use has now extended into most areas of cell biology and biochemistry [4,5]. Since 1979 biennial International Conferences on Partitioning in Aqueous Two-Phase Systems have been held in Los Angeles (1979), Sheffield (1981), Vancouver (1983) and Lund (1985). The 5th conference was held in Oxford from 23-28 August 1987 and was entitled "Advances in Separations Using Aqueous Phase Systems in Cell Biology and Biotechnology". It is the formal presentations from this meeting which comprise this volume. In contrast to earlier books on phase partitioning [4,5] this volume contains, for the first time, worldwide contributions from over sixty partitioners from a variety of scientific disciplines, thereby providing a detailed overview of the widespread application and potential of bioseparations using phase partitioning. Disciplines include Biophysics, Biochemistry, Cell Biology, Microbiology, Biotechnology and Process Engineering, in both academic and commercial establishments. These biennial conferences allow advances in these diverse partitioning fields to be reviewed and compared; they also provide an opportunity for those considering using phase partitioning to obtain information, advice and contacts. Attendance has grown steadily over the years and 140 scientists came to Oxford. The conference consisted of ten symposia on areas of application of partitioning which have been organised as specific chapters in this volume.