Prominent progress in molecular biology was only made when it became possible to separate functionally distinct molecules by taking advantage of their biophysical properties. Likewise, the analysis of the functions of hetero- geneous populations of immunocompetent cells, as to the functional properties of their various subpopulations, can not be done until these can be isolated in reasonably pure form by selective fractionation. During the last few years significant advances have been made in this field, and cells have been separated according to size, density or charge (MILLER et aI., 1969;...