Studies of the immunochemical specificity of antigen-induced thymidine-2-(14)C incorporation in lymph node cells obtained from animals immunized to a series of closely related alpha-DNP-oligolysines, epsilon-DNP-oligolysines, and oligolysines have shown that the sensitized cell exhibits an extraordinary degree of specificity for antigen. The sensitized cell is maximally stimulated by the homologous immunizing antigen and can discriminate among compounds which differ from one another only in the position of a dinitrophenyl group or D-lysine residue on an identical oligolysine backbone. These studies support the view that the immunogen is not degraded prior to the induction of the immune response, and that the majority of cells produced as a consequence of immunization have stereospecific antigen receptors for the DNP-oligolysine used to induce the response; a smaller and more variably sized population of cells is produced with receptors specific for the oligolysine portion of the immunizing antigen. When specifically sensitized lymph node cell cultures are stimulated in vitro by heterologous DNP-oligolysines, the oligolysine- and not the DNP-oligolysine-sensitive population of cells appears to play a crucial role in the specificity of such cross-reactions. It is concluded from these studies that the antigen receptor on the sensitized lymph node cell differs in both kind and degree from conventional antibody. The chemical nature of the receptor and the means by which this receptor reacts with antigen to initiate the biosynthetic or proliferative cellular immune response still remain undefined.