The numbers of antigen-reactive cells (ARC) responding to a purified protein, the polymer of S. adelaide flagellin, have been assayed in cell populations derived from several lymphoid tissues of mice. The assay, which employs the cell transfer into lethally irradiated mice, indicates that there is a response of ARC in bone marrow in the absence of thymus cells. This suggests that the immune response to this protein antigen is not thymus dependent. The presence of relatively large numbers of ARC in Peyer's patches argues for their direct participation in the immune response in the adult mouse. The kinetics of ARC and antibody-forming cells in the early primary response employing the transfer system is described. The numbers of ARC declined during the first 2 days of the immune response, but by day 6 had increased to about five times the number in unprimed spleen cells. The rise is believed to be a result of the primary injection of antigen and therefore may be described as memory; however, these experiments have not been able to further elucidate any specific qualities of the "memory cell." Tolerance induction in C57BL/Brad mice produced by repeated injections of a cyanogen bromide digest of the antigen is described. The ARC or its precursor is shown to be the site of the lesion of tolerance by direct investigation
