The immunological response in vivo to a series of size-fractionated linear polymers of acrylamide substituted with hapten has been measured in mice. A sharp threshold was observed in immunogenic response elicited by various polymer preparations. All polymers with less than 12 to 16 appropriately spaced hapten groups per molecule were nonimmunogenic, while those polymers with greater than this number were fully immunogenic. The results lead to the conclusion that the immunological response at its most elementary level is quantized, i.e., a minimum specific number of antigen receptors (approximately 12 to 16) must be connected together as a spatially continuous cluster, an immunon, before an immunogenic signal is delivered to the responding cell.