The rate of antigen binding by mouse lymphoid cells has been investigated with polymerized flagellin of Salmonella adelaide that had been biosynthetically labeled with tritium. Autoradiographs of lymphnode cells incubated with the tritiated antigen at 37 degrees showed aggregation of antigen receptors to one cell pole. This was followed 4-5 hr later by the appearance of antigen receptors on the cell surface, at a density severalfold higher than at the time of first contact with the antigen. Antigen-binding cells exposed in vitro to antigen concentrations known to cause high zone tolerance induction failed to form polar antigen caps once they had entered the phase of increased receptor formation. The data suggest that sufficient crosslinking of receptors by the antigen to cause their aggregation triggers the cell to differentiate and to increase its density of antigen receptors. In the presence of antigen concentrations favoring high zone tolerance, receptors may become interlinked to such an extent that they are prevented from aggregation. Such a "frozen" state of the antigen recognition system would render the cell unresponsive to antigenic stimuli.