Included in this volume is a broad range of topics. Immunology is such a diverse field that many of the subspecialties overlap, and one finds it convenient and necessary to integrate information from several of them. We try to focus on the molecular aspects of immunology as much as is reasonable, but some con­ tributions consist of ablend of molecular and cellular immunology and even immunopathology. This is as it should be, since information at the molecular level often provides an explanation of phenomena observed at other levels. Myelin basic protein holds the interest of immunologists because it is impli­ cated in the induction of the autoimmune disease called experimental allergie encephalomyelitis (EAE). Although much biochemical and immunological information about this protein has been uncovered, it is not understood how such an inaccessible self-antigen can serve as the focal point in the central ner­ vous system for myelin basic protein-specific EAE-inducing T cells. Day dis­ cusses the problem by first reviewing the sequences of the proteins from several species and the antigenicity of the proteins and peptides derived from them. The reader is then led into a thorough discussion of the immunological relation­ ships that do and do not influence development of the encephalitis. From this discussion, the author promulgates the bystander model as the best overall mechanism to explain why different fragments of the highly conserved protein are needed by various species to give rise to the same type of localized central nervous system disease.