In the normal state, immunoglobulins are polyclonal. They migrate as a broad band on electrophoresis, reflecting the diversity of immunoglobulin species secreted by the many clones of immunoglobulin-producing cells. In lymphoproliferative disorders, monoclonal immunoglobulins are often found, migrating as electrophoretically homogeneous bands, reflecting the large amount of a single immunoglobulin being secreted by the single proliferating neoplastic clone. In certain neurological diseases, such as multiple sclerosis, the cerebrospinal fluid (CSF)2 often exhibits an oligoclonal immunoglobulin pattern consisting of several electrophoretically homogeneous bands in the gamma-globulin region (Figure 1). We present two cases of neurological disease with an ohgoclonal immunoglobulin pattern in the CSF electropherogram. In the discussion we consider the definition, etiology, and pathogenesis of multiple sclerosis, and laboratory tests that are useful in multiple sclerosis, emphasizing methods for the demonstration and interpretation of an oligoclonal immunoglobulin pattern.