In order to test the hypothesis that the immune system plays a role in the pathogenesis of amyotrophic lateral sclerosis (ALS), the cellular composition of the spinal cord inflammatory infiltrate was analysed in eight cases of sporadic ALS by a panel of monoclonal antibodies. The majority of the many diffusely scattered lymphocytes seen in the anterior and lateral corticospinal tracts and anterior horns belonged to the suppressor/cytotoxic T-cell subset and were admixed with variable numbers of macrophages. Helper-inducer T-cells were rare and B-cells were conspicuously absent. Compared to controls, ALS specimens exhibited an increase in major histocompatibility complex (MHC) products or human leucocyte antigens (HLA) in the corticospinal tracts and anterior horns. HLA-ABC antigens were expressed in the honeycomb pattern of the glial matrix of the spinal cord, and HLA-DR antigens were strongly expressed by large dendritic cells. In addition, macrophages and endothelial cells were labelled by HLA-DR. These findings suggest that an autoimmune process or infectious agent may play a role in ALS.