Type II cells were examined in situ in order to study the role played by actin filaments in secretion of surfactant. Following β-adrenergic stimulation, the number of lamellar bodies per cell profile decreased substantially, an indication that exocytosis had occurred. In isoproterenol-stimulated cells decoration of actin by S1 revealed greater numbers of filaments associated with lamellar bodies, free in the cytoplasm, and associated with the cellular apex than in control cells. If cells were treated prior to β-agonist stimulation with cytochalasin D, a drug which disrupts filamentous actin, the secretory response to the agonist was abolished. Together these two observations suggest that actin filaments participate in the intracellular movement and/or extrusion of lamellar bodies from the cells.