Abstract T cells respond to foreign antigen only when the latter is presented on the surface of an antigen-presenting cell (APC) together with a molecule encoded in the major histocompatibility complex (MHC). The nature of this antigen presentation is poorly understood. The difficulty of demonstrating soluble antigen serologically on the surface of APC, the finding that in some cases peptides of a certain protein are more antigenic than the whole molecule [1], the Observation that T cells respond to native and denatured antigen equally well irrespective of which form was used for priming [2], and the fact that cells can rapidly degrade the antigen have led to the concept of antigen processing. According to this hypothesis the antigen is internalized and structurally altered (possibly enzymatically degraded) by the APC, and is then redisplayed on the surface of this cell in association with MHC molecules. Only antigens thus converted are recognizable by T cells [3].