Volume 3 continues the approach carried out in the first two volumes of this se ries of publishing articles on membrane methodology which include, in addition to procedural details, incisive discussions of the ap­ plications of the methods and of their limitations. Wh at is the theoretical basis of the method, how and to what problems can it be applied, how does one interpret the results, what has thus far been achieved by the method, what lies in the future-these are the questions the authors have tried to answer. No area of membrane biology engages the interest of more investigators than studies of the plasma membrane. Four chapters in this volume are concerned with one or more aspects of the cell surface. Fundamental to all studies of the cell surface are the isolation and characterization of pure plasma membranes. Many preparations described in the literature are inadequate or are inadequately characterized. In the first chapter, Neville discusses the theoretical and practical bases of tissue fractionation, empha­ sizes the variations in enzyme content among plasma membranes from different sources, offers guidance in the choice of the proper criteria for assessing membrane purity, and suggests the best markers for detecting the possible presence of contaminating organelles. To review in detail each of the many preparations of plasma membranes that have been published is impossible.