to investigate the possibility of local antigen presentation within the peripheral nerve in amyotrophic lateral sclerosis (als), cryostat sections of 83 peripheral nerve biopsies were stained for the demonstration of hla-dr using a monoclonal antibody. forty samples showed increased expression of hla-dr in endoneurium. the phenotypic characteristics of the hla-dr positive cells are chiefly schwann cells, using s-100 protein as a marker. we did not detect any co-expression between hla-dr and nf (axons) and hla-dr and myelin marker. we also detected co-expression between hla-dr and ngfr in a majority of hla-dr positive cells. inflammatory cells were infrequent, being detected only in 11 cases, predominantly around epineurial blood vessels. motor and sensory nerve biopsies performed simultaneously showed higher expression of hla-dr in motor nerves in 2 out of 4 patients. the significance of these findings is not clear. the presence of endoneurial cells expressing hla-dr suggests that an autoimmune mechanism may be involved in als having schwann as the main target.