The object of the proposed investigation is to test the following hypothesis: Long-lived, non-dividing, recirculating lymphocytes can initiate the secondary antibody response in rats. Some of these cells are "non-thymus derived" and are the precursors of the antibody forming cells. Others are "thymus derived" and augment the response produced by the "non-thymus derived" cells by binding antigen and presenting it efficiently to them. Short-lived, non-recirculating, "non-thymus derived" lymphocytes are the precursors of the antibody forming cells in the primary response. "Thymus-derived" cells similar to those above also augment this response. In order to study this model, purified populations of the types of lymphocytes described above will be tested for their ability to restore the adoptive primary and secondary antibody responses of X-irradiated rats to proteins and to hapten-protein conjugates.