Laminin-1 is an extracellular matrix protein composed of three polypeptide chains that are designated α1, βT1, and γ1. We investigated the expression of laminin α1, β1, and γ1 subunit chains during several stages of rabbit fetal lung development. Utilizing polyclonal antibodies directed against human placental laminin and immunoblot analysis, we found that the highest levels of laminin α1, β1, and γ1 subunit chains in the fetal lung were present on day 26 of gestation (term = 31 days), coincident with the initiation of alveolar epithelial cell differentiation. Levels of the laminin chains were approximately five times higher in fetal lung at day 26 of gestation than in adult lung tissue. Different temporal patterns of laminin α1, β1, and γ1 subunit chain expression were observed, data suggestive that the chains are independently regulated during lung development. Laminin was localized to the basement membranes of bronchi, bronchioles, preal-veolar ducts, and blood vessels in fetal lung tissue, as shown by immunostaining with polyclonal laminin antibodies. A similar staining pattern was observed in adult lung tissue, but the alveolar wall was also stained. Laminin was also observed surrounding a few mesenchymal cells in fetal lung on day 19 of gestation; the number of positive mesenchymal cells increased with lung development. Laminin α1 subunit chains, detected using a monoclonal antibody, were present in the basement membranes of bronchi, bronchioles, preal-veolar ducts, and blood vessels in fetal lung tissue. No laminin α1 chain staining was observed in the mesenchyme of early fetal lung tissue. Using a monoclonal antibody, laminin β1 subunit chains were immunolocalized in the basement membranes of bronchi, bronchioles, in prealveolar ducts, and surrounding some mesenchymal cells in fetal lung tissue. Laminin α1 and β1 subunit chains in adult lung tissue were present in basement membranes of airways, blood vessels, and alveoli. Thus, changes in the localization and accumulation of laminin near the time of alveolar type I and type II epithelial cell differentiation suggest that laminin may play a role in mediating the differentiation of these cell types during rabbit fetal lung development. ©1995 Wiley-Liss, Inc.