The formation of elastic fiber network in the lung is develop-mentally regulated. In this study we first demonstrated that tropoelastin mRNA per unit total RNA in the fetal bovine lung increased from 110 to 250 days of gestation (270 day term) as measured by Northern blot analysis. To examine the extent that smooth muscle (SM) type cells contribute to this gestational increase in elastin phenotype, we utilized a dual immunofluorescent staining technique on lung sections with anti-elastin polyclonal and anti-SM isoactin monoclonal antibodies. Elastin staining was always found to localize in proximity to SM isoactin-positive cells at various stages of prenatal lung parenchymal development. Minimal, if any, elastin was seen at interstitial fibroblasts, which were negative for the SM isoactin staining. Distribution of SM (type) cells and elastic fiber together along the airways became sparse and discontinuous distally, and it seemed that formation of air sacs was between the discontinuous elastic fibers. We speculate that smooth muscle (type) cells may be the major elastogenic cells of distal airways and may play an important role in alveolar formation.