The physiological role of pulmonary surfactant is probably not limited to conferring mechanical stability to the alveoli. Increasing evidence suggests that surfactant components, in particular the hydrophilic surfactant proteins SP-A and SP-D, play potentially important roles in host defense mechanisms. Both SP-A and SP-D are collagenous C-type lectins (collectins) that are structurally and, perhaps, functionally related to collectins in the circulation. As will be discussed in this review, evidence is accumulating that the alveolar collectins SP-A and SP-D could be important components of a first-line defense system against infiltrating pathogenic micro-organisms and viruses.