Neuronal microtubule-associated proteins (MAPs) are important components of neurons and are believed to regulate neuronal function and development by controlling the assembly of microtubules and the interaction of microtubules with other cytoplasmic organelles. We studied the immunohistochemical localization of MAPs 1, 2, 5, and tau in the intestinal tissues of five patients with Hirschsprung's disease and in five normal controls using monoclonal antibodies. Microtubule-associated proteins 5 and tau proved to be excellent enteric neuronal markers; they were specifically located in the nerve cell bodies and processes of normal intestine as well as in the abnormal hypertrophied nerve fibers of aganglionic colon. Fine fibrillar structures in the neuroplasm were revealed in greater detail than were those obtained from studies with conventional markers, including neuron-specific enolase, S-100 protein, and neurofilament protein. A slight reduction of MAPs 5 and tau immunoreactivity was observed in the aganglionic colon compared with normal colon. Microtubule-associated proteins 1 and 2 were absent from the nerve fibers in both normal and aganglionic colon. This study suggests that immunostaining for MAPs 5 and tau may be superior to other immunohistochemical methods for diagnosing Hirschsprung's disease; however, in view of its limited retrospective nature these findings need to be corroborated by a large prospective evaluation.