The occurrence of IgA subclasses in pathological conditions of the nervous system was studied by means of monoclonal antibodies and an indirect immunofluorescence technique. IgA1- and/or IgA2-positive lymphoid (plasma) cells were found in demyelinating diseases comprising multiple sclerosis, Guillain-Barré syndrome, and adrenoleukodystrophy, in various inflammatory diseases, and in tumors, some of which exhibited labeling of tumor cells. Demyelinating and inflammatory diseases with chronic course displayed some prevalence of IgA2-, and tumors some prevalence of IgA1-positive cells. This is the first demonstration of IgA1 and IgA2 in the nervous system.