Surfactant protein (SP) A and SP-D are the pulmonary members of the collectin family, structurally related proteins involved in innate immune responses. Here, we have examined the abilities of SP-A, SP-D, mannose-binding protein (MBP), and the complement component C1q to stimulate actin-based cellular functions in rat alveolar macrophages and peripheral blood monocytes. Our goal in this study was to examine the cell specificity of the effects of the collectins to understand further the mechanisms by which SP-A and SP-D stimulate alveolar macrophages. We found that SP-A and SP-D have lung cell-specific effects at physiologically relevant concentrations; they stimulate directional actin polymerization and chemotaxis in alveolar macrophages but not in monocytes. Although C1q and MBP weakly stimulate the rearrangement of actin in both cell types, C1q is chemotactic only for peripheral blood monocytes and MBP does not stimulate chemotaxis of either cell type. Neither C1q nor MBP stimulates actin polymerization in alveolar macrophages. These results support the hypothesis that alveolar macrophages express receptors specific for the pulmonary collectins SP-A and SP-D and provide insight into the potential roles of collectins in the recruitment and maturation of mononuclear phagocytes in the lung.