Recent advances in the understanding of the immunology of multiple sclerosis (MS) are discussed. In the central nervous system (CNS) major histocompatibility complex (MHC) class I and II antigens, and lymphocyte adhesion molecules are expressed. Furthermore, there are cells present which produce important effector molecules such as interferon (IFN)-gamma and tumor necrosis factor (TNF). A role for myelin antigen-specific autoimmunity is supported, but not proven, by the presence of a marked quantitative increase of T and B cells with such specificities, enriched to the CNS. Several myelin antigens, as well as a multitude of peptide parts of these, are apparently recognized. It will be of interest to study the functional abilities of these cells and how they are regulated.