BACKGROUND The lung develops by epithelial tubes budding and branching into a flexible mesenchyme. This growth is associated with the remodelling of the epithelial basement membrane, of which laminin is a major component. METHODS Both the synthesis and expression of laminin were studied in the human lung between 10 and 31 weeks of gestation, using in situ hybridization and immunohistochemistry. RESULTS The synthesis of the beta chain was active in the epithelial and surrounding mesenchymal cells. The mRNAs coding for the gamma chain were less abundant and mainly found in the epithelium. The synthesis of these two chains continued throughout gestation, and no significant difference in the density of hybridization grains could be detected between the tips of the expanding buds and the proximal portions. Immunohistochemical localization of laminin showed important modifications of the basement membrane during gestation. In the first part of the pseudoglandular stage the epithelial basement membrane stained continuously for laminin. Later, the basement membrane was labelled in a graded fashion: at the apex of the growing buds the staining became weak with focal disruptions. Both epithelial and mesenchymal synthesis of laminin remained active, while the polypeptide was undetectable using immunohistochemistry. CONCLUSIONS These findings suggest that the remodelling of the basement membrane during human lung morphogenesis is probably not related to a decreasing synthesis of laminin, but to either a proteolytic degradation or the assembly of an inadequate complex undetectable with the polyclonal antibody antilaminin.