A new reliable immunohistochemical method for diagnosing Hirschsprung's disease (HD) using our unique monoclonal antibody (MAb) 171B5 against synaptic vesicles is described. Fresh frozen sections of rectal tissues were used from 13 patients with HD aged 2 weeks to 13 months; 9 had rectosigmoid HD and 4 had total colonic aganglionosis (TCA). Comparable normal colonic and rectal specimens were also obtained from 13 age-matched controls. All specimens were labeled with MAb 171B5, to demonstrate neuronal innervation patterns of both mucosa and submucosa. In all control specimens, many synapses arranged in variciform plexuses were seen in the lamina propria, a moderate number in the muscularis mucosae, and dense clusters in the submucosal plexus. In all aganglionic specimens, only scanty numbers of synapses which were not organized in variciform plexuses were seen in the lamina propria, none in the muscularis mucosae, and a few in the submucosa. These findings suggest that MAb 171B5 immunohistochemistry on the lamina propria alone can differentiate between normal and aganglionic bowel and appears to be a reliable and useful method for detecting HD on suction rectal biopsy.