Four separate effects can be demonstrated when lymphoid cell suspensions are passed through columns of siliconed glass beads. (a) A temperature-dependent "active adherence" of phagocytic cells, such as macrophages and polymorphs. (b) A temperature-independent and selective trapping by "physical adherence" of particular classes of lymphoid cells, including certain antibody-forming cells. (c) A "size-filtration" effect that traps larger cells, but only becomes significant with beads below 100 µ in diameter. (d) A selective retention of damaged cells, which occurs with all columns under all conditions tested. An active adherence column technique has been developed to separate phagocytes from lymphocytes while minimizing selection within the lymphocyte population by physical adherence or size filtration. In less than 10 min at 37°C it reproducibly produces a preparation of mouse spleen lymphocytes >500-fold depleted of active macrophages, and approximately 50-fold depleted of active polymorphs, with good over-all cell recoveries and cell viability. The lymphocyte fraction appears fully active in its ability to initiate immune responses to at least two different antigens, but is changed in over-all composition and selectively depleted in certain classes of antibody-forming cells.