A 14 kDa basic protein isolated from rat lung lavage was demonstrated to be lysozyme by its amino acid sequence analysis. An antiserum to rat lysozyme stained type II pneumocytes and alveolar macrophages. In rat lungs, no staining of the airway cells was noted. Lysozyme was detectable in type II pneumocytes by immunocytochemistry and by a quantitative immunoassay of lung homogenates of fetal lungs at Day 20 of gestation. An increase in the lysozyme content of the lung with increasing gestational and postnatal age of the rat was noted. In adult animals, lysozyme accounts for about 169.0 µg/g of wet lung weight and 0.3% of the soluble proteins in lung homogenate. Lysozyme constitutes about 6.6% of the total soluble proteins in rat lung lavage. Metabolic labeling and immunoprecipitation were used to demonstrate that rat type II pneumocytes synthesize and secrete lysozyme in vitro. However, in human lungs, lysozyme was identified in serous submucosal glands but not in alveolar type II pneumocytes. The results demonstrate differential distribution of a secretory protein in rodent and human lungs and indicate that in the rat lysozyme could be used as an immunohistologic marker for type II pneumocytes and as an indicator of secretory activity and maturation of type II pneumocytes.