Previous immunohistochemical studies have shown an abnormal distribution of extracellular matrix (ECM) proteins, including laminin, in the smooth muscle layer of muscularis externa in Hirschsprung's disease (HD) bowel. These findings supported the hypothesis that an abnormal ECM microenvironment may be responsible for the failure of migration and/or development of the neural crest cells in the gut in HD. In order to determine the cause of the abnormality in laminin distribution, solid-phase enzyme-linked immunosorbent assays and immunoblots were used to quantitate the ECM protein laminin and characterize its subunits, respectively, in extracts of the dissected smooth muscle layer of the muscularis externa. In the aganglionic bowel, laminin (median concentration, 32.4 ng/mg of tissue) was found to be present in significantly greater quantity than in both the normoganglionic bowel of the same specimen (median, 17.2 ng/mg, P less than or equal to .05) and the normal bowel of age-matched controls (median, 9.7 ng/mg, P less than or equal to .05). Laminin concentration was also found to be significantly higher in normoganglionic HD bowel (median, 17.2 ng/mg) than in age-matched control specimens (median, 10.8 ng/mg, P less than or equal to .05). No difference was observed in the subunit composition of laminin in HD and control extracts analysed by immunoblot after polyacrylamide gel electrophoresis. This study demonstrates a quantitative abnormality of laminin in the bowel in HD, supporting the hypothesis that "abnormal microenvironment" may have a role in the pathogenesis of HD.