The technology for the separation of discrete types of cells from complex cell populations is a recent development. It is perhaps surprising that methods of fractionating and purifying subcellular particles, macromolecules, and enzymes were devised long before whole cells could reliably be separated from each other. In part the ability to clone and grow homogeneous populations of microorgan isms or animal cell lines bypassed this need. However, in part the problem was simply ignored, whole tissues being homogenized and treated as if their cellular constituents were all identical. The main impetus for devising cell-separation pro cedures has been the increasing interest in animal cell differentiation, in particular in the development of blood elements from hemopoietic stem cells, and in the behavior of lymphoid cells during the immune response. In these cases the sepa ration procedures serve a dual purpose. The hemopoietic and lymphoid tissues studied are extremely complex cell mixtures, so there is a need to isolate or to purify the particular cell under study. In addition, investigations are in a realm where conventional cell morphology is often inadequate; to overcome this limita tion cell-separation procedures are used analytically, to help characterize and define new functional classes of cells. These two aspects, preparative and analytical, are emphasized in this review. The procedures discussed generally meet the following criteria: (i) the theoretical basis of the separation is established; (ii) the procedure represents an efficient application of the basic separation principle; (iii) the distribution in fractions re flects inherent properties of the cells, and is not an artifact of the system; (iv) the procedure IS reproducible and quantitative, so cells can be characterized by their separation behavior; (v) an adequate number of cells can be processed; (vi) over all recovery of viable cells is high. This may seem an unnecessary emphasis on