Pulmonary surfactant functions in the lungs at the interface between the air and the liquid layer that covers the epithelial surface. Pulmonary surfactant is functionally, chemically, and morphologically heterogeneous. Although its best-defined function (and the one for which it is named) is to lower surface tension at the air/liquid interface within the lung, pulmonary surfactant is now believed to have additional functions, including mediating host defense against infection (100, 103). Chemically, surfactant consists of lip. ds (pri marily phospholipids) and four specific surfactant-associated proteins: SP-A, SP-B, SP-C, and SP-D (reviewed by Hawgood & Shiffer, this volume). Several morphologic forms of surfactant have been identified in alveolar liquid in fetal lungs and in fluid obtained by endobronchial lavage (see 113 for review). Our understanding of the precise function of each of these forms as well as an accurate chemical characterization of them is incomplete.