Postulated need for a surfactant-containing, submicroscopic layer lining mammalian lung alveoli has stimulated electron microscopic search for this layer. Several investigators who performed ultrastructural studies reported finding an alveolar layer. Some studies in which multivalent cationic "stains" were used revealed an adherent layer on alveolar epithelial cells. This is probably a mucopolysaccharide-containing cell coat common to most cells. Other studies illustrated thick homogeneous layers. These may result from blood protein leakage into alveolar air spaces during tissue preparation. Still other studies revealed noncontinuous, single or multiple, osmiophilic, thin membranes. These may represent the sought-for alveolar lining layer. Newer methods using tissue freezing offer promise for resolving the question of layer presence. Convincing ultrastructural evidence for a continuous surfactant-containing layer on the alveolar surface has not yet been obtained.