Antigen degradation and peptide loading to major histocompatibility complex class I and class II molecules are described with special emphasis on ''noncanonical'' pathways. Examples of specific peptide loading for measles proteins are provided. In addition, characterization of defined epitopes presented to T cells can lead to the design of products of special interest in medicine and, in particular, in development of vaccines. (C) American Society for Histocompatibility and Immunogenetics, 1997.