Laminins are trimeric glycoprotein components of basement membranes. Each laminin has three structurally similar chains, designated α , β , and γ . Five laminin α chains are now known. In previous studies using monoclonal antibody 4C7, laminin α 1 was thought to be present in basement membranes of human lung throughout development and in the adult, but recent expression studies have demonstrated that 4C7 identifies laminin α 5 rather than α 1. To determine the temporal and spatial patterns of laminin α 1 and laminin α 5 in developing human lung, we prepared complementary DNA probes specific for laminin α 1 and α 5 messenger RNAs (mRNAs). By Northern analysis, laminin α 1 mRNA was prominent in first-trimester fetal lung, but was not detectable at 23 wk or at later times. In contrast, laminin α 5 mRNA was readily detected in early fetal lung and remained present thereafter. Immunohistochemical staining demonstrated laminin α 1 only in early fetal lung, whereas laminin α 5 was persistent from the early fetal period. In situ hybridization localized laminin α 1 expression to distal epithelium in the first-trimester lung, and laminin α 5 to all epithelium and developing pulmonary arteries from the first trimester through the perinatal period. These studies indicate that laminin α 1 expression is restricted to early human lung morphogenesis, whereas the expression of laminin α 5 in human lung is continuous from early lung development through adult life. It is evident that laminin α 1 and laminin α 5 have different roles in the development of the human lung.