We have asked the question: how do partner cells in immunoregulatory interactions between T cell subsets acquire the ability to recognize and react appropriately with one another? In particular, we have asked whether these communication events are completely determined by the cell's genetic constitution, or whether the recognition events can be learned during ontogeny. We have found that the T cells of parent into F1 chimeras and homozygous nude mice with F1 thymus grafts not only learn to react with genetically disparate acceptor cells, but that the receptors on the acceptor cells themselves learn to react with genetically disparate producer cells. This learning process can overcome both major histocompatibility complex- and immunoglobulin heavy chain variable region-linked restricted communication between T cell subsets. We interpret these findings to indicate that thymic elements can start a cascade of differential events. The thymic elements, whether endogenous or passively acquired, select from a pool of producer cells those that will react appropriately with the thymic selecting cells, and these cells become expanded. Then, the private markers (idiotype) on the expanded pool of producer cells act as selecting and expanding elements that choose from a pool of acceptor cells those that recognize the producer cells idiotype as self. This second differentiational event, although apparently thymus evidence that this type of acceptor cell differentiation could also take place during the course of an immune response.