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Saturated phospholipids are known to be the only surface active compounds present in the surfactant system of the lung. Using light microscopy, the identification in situ of pulmonary surfactant has always been hampered by the lack of satisfactory fixatives and dyes which act on saturated phospholipids fast enough to prevent the complete loss of surfactant in the solutions. In this study we adopted the tricomplex flocculation proposed by Elbers et al. (1965) to fix surfactant phospholipids on frozen sections obtained from human, pig and rat lungs. Small pieces of lung tissue were quickly frozen in freon 22 kept at -75 C.; eight micron sections were cut in a cryostat, air dried and immersed for 30s-5m in a 0.05 N Pb(NO3)2 + K3Fe (CN)6 solution in 10% formalin. Lead ions bound to the choline portion of phospholipid molecules were subsequently revealed in a 30 mM ammonium sulfide solution. This procedure delineates a dark brown filmy structure in the respiratory parenchyma, which is very loosely attached to the alveoli and appears to be related to lung surfactant. Preliminary lung lavage or pretreatment of sections with saline, aldehyde fixatives and several organic solvents, fully or partially abolish the stain.