Normal rabbit bone marrow cells have been studied according to their immunological reactivity in vitro. The test system involved stimulation by antigen after the subsequent stimulation into cellular proliferation by measuring the uptake of tritium-labeled thymidine. Specific separation of immunological reactivity was obtained by filtration of cells through antigen-coated bead columns. All experimental evidence supported the view that this separation was due to the existence of preformed antibody molecules on the outer cell surface of the antigen-recognizing cells. The response to antigenic stimulation was shown to be strictly dose related and, using supraoptimal concentrations of one antigen, no increased DNA synthesis was recorded. That this state of unresponsiveness represented a state of immunological paralysis was indicated by the normal response of these cells to stimulation by a second antigen in optimal concentration. Thus both methods, cell separation on antigen-coated columns or induction of specific unresponsiveness by antigen in vitro, can produce a cell population specifically devoid of cells reactive against a given antigen.