Morphological work dealing with correlation between structure and function necessitates careful fixation technique to ensure preservation of the lung under well defined, nearly physiological conditions. When the problems, merits, and disadvantages of two techniques-(1) fixation "from the front" by instillation of fixatives into airways and (2) fixation "from behind" by vascular perfusion into the pulmonary artery—are discussed, it is concluded that perfusion-fixation is the only one presently available for solving some special problems such as (a) preservation of alveolar lining layer or alveolar edema, (b) fixation of lungs at different volume histories, or (c) preservation of the fluid lining the airways. This method is particularly useful for studies related to air pollution. Lung surfactant has definite morphological features. "Tubular myelin figures" may represent surface-active lipoprotein.