The recent establishment of a role for laminin in mouse lung organogenesis (Schuger et al., 1990a,b, 1991) prompted us to study its expression in the developing lung. Laminin A and B chains were detected in the murine lung from the first hours of development onward. In situ hybridization of mRNA as well as SDS‐PAGE studies of lung cells in monoculture indicated that both epithelium and mesenchyme produce complete laminin molecules. Quantitative analysis of the in situ hybridization studies showed a gradual increase in laminin expression during development which was further supported by immunohistochemistry and ELISA. The overall pattern of expression suggested that the effects of laminin in morphogenesis were not restricted to a particular stage of development. Furthermore, the increase in expression during late development supported a role for the molecule in the fetal lung, which was not previously established. We next determined whether the increase in laminin production modulated the behavior of fetal lung cells as compared with their embryonic counterparts. We previously showed that organotypic pattern formation does not occur in cultures of mixed embryonic lung cells unless exogenous laminin is added (Schuger et al., 1990b). Organotypic pattern formation is the result of cell sorting into epithelial and mesenchymal compartments and further rearrangement in a pattern resembling the tissue of origin. In the present study, we demonstrated that organotypic pattern formation occurs spontaneously in cultures of mixed fetal lung cells, which express high laminin levels. Pattern formation was abolished by antibodies to laminin. These studies suggest a correlation between laminin expression and the ability of lung cells in culture to reproduce normal tissue patterns. We conclude that laminin is critical for epithelial‐mesenchymal recognition and further morphogenic interaction during both the embryonic and fetal stages of lung development. © 1992 Wiley‐Liss, Inc.