We have developed a simple method for the fractionation of T and B lymphocytes. Plastic dishes coated with antibodies specific for mouse Ig selectively bind splenic B lymphocytes. The adherent cells are easily removed by gentle pipetting; both adherent and nonadherent populations retain immunologic function. In a typical experiment, when 3 X 10(7) splenic lymphocytes were added to a 100 X 15 mm plastic dish coated with microgram quantities of anti-Ig, 98 % of the nonadherent cells were Ig negative and 97% of the adherent cells were Ig positive. The method is sufficiently sensitive to allow detection and separation of cell types comprising as little as 2% of the total population and can be modified to allow the selection of cells by a double-antibody procedure. We believe that the plastic dish method will be generally useful for fractionating cells on the basis of their cell surface antigens.