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Cells of the different lymphoid organs in the normal adult rabbit were investigated for their capacity to respond in vitro to a number of stimuli, such as phytohemagglutinin (PHA), anti-rabbit immunoglobulin antiserum (GARIG) and allogeneic and xenogeneic lymphoid cells, and for their capacity to adsorb radioactively-labeled anti-immunoglobulin antiserum. The bone marrow cells responded minimally to PHA, GARIG, and the allogeneic and xenogeneic stimuli. The thymus cells were unable to respond to stimulation with GARIG although they responded to the other stimuli. The cells of the other lymphoid organs tested responded to all the mitogenic agents, to varying degrees. On the basis of the results presented and the findings of other investigators, it is concluded that: 1. The response of the cells to GARIG indicates a potential capacity to mediate humoral immunity and requires the presence of immunoglobulin or immunoglobulin-like recognition sites on the cell surface. 2. The response of the cells to PHA and allogeneic and xenogeneic cells indicates a potential capacity to mediate cellular immunity and does not necessitate the presence of immunoglobulin-recognition sites on the cell surface. 3. The thymus in the normal adult rabbit consists of cells capable of mediating cellular immunity only. 4. The other lymphoid organs appear to possess cells capable of mediating humoral and cellular immunity.

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In a preceding article, evidence was presented for the existence of two distinct populations of lymphocytes in the normal adult r a b b i t --o n e capable of responding to stimulation with phytohemagglutinin (PHA) 1 and the other to goat antiserum to rabbit immunoglobulins (1). The question which these results pose is whether one or both of these functionally different populations of cells are capable of reacting to common protein antigens. This invesigation was designed to provide the answer to this question. The results clearly imply that only the lymphocytes which react to anti-immunoglobulin antiserum react to stimulation with specific antigen, with both of these reagents probably interacting with the same receptor site on the lymphocyte surface.

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