A study was made of antibody-forming cell precursors (B cells) and thymus-derived helper cells (T cells) of mouse spleen filtered through nylon wool columns. Two populations of spleen cells were separated: effluent cells nonadhering to the nylon fibers, and nylon-adherent cells which could be eluted from the columns by mechanical agitation. The presence or absence of functional lymphocytes was demonstrated by transplantation into irradiated syngeneic recipients, challenge with antigen, and assay for specific antibody-forming cells. By transplanting each of the column-filtered subpopulations separately, in combination, and with either bone marrow or thymus cells (known sources of B or T lymphocytes, respectively), it was possible to establish that B and T cells were separated by the column filtration; the effluent population was enriched with functional helper cells and depleted of precursor cells, whereas the nylon-adherent population was enriched with competent precursors and depleted of helpers.