To summarize, during the period under review there have been considerable advances in our understanding of how antigen is associated with MHC on the surface of a presenting cell. Basic rules which govern this association have been confirmed as including both the nature and the configuration of the antigen. Although the different processing pathways which generate the antigen-MHC complex have remained ill-defined, it is now much more clear what the optimum end point of such processing must be. As a result of these studies, several new questions can now be addressed in the field of antigen presentation. During the next few years it should become apparent why there should be such an unusual differential presenting cell requirement for T cells which are expressing a selected variable region of the T cell receptor and whether other T cells expressing different receptors have different optimum presentation requirements. It should become clear also whether different antigens have similar residues at the sides of the alpha helix which are important in MHC binding, and whether the sheet or helix models are more important in vivo. A rational explanation for dominance of alloreactivity has been proposed, and kinetic studies of self-MHC processing are essential to define this further. Furthermore the possibility of an antigen unmasking an MHC polymorphism (which in turn could influence presentation) will have to be considered seriously in the pathogenesis of auto-immunity.(ABSTRACT TRUNCATED AT 250 WORDS)