Cultured alveolar type II cells and alveolar macrophages were found to secrete colony-stimulating factors (CSF) into the medium. Surfactant protein A (SP-A; 0.1-5 micrograms/ml) and bacterial lipopolysaccharide (LPS; 10-20 micrograms/ml) were found to upregulate the secretion of CSF (seven-fold) from these cells. However, a reversal of the stimulatory effect was observed when the two agents were added simultaneously to the cells. SP-A-enhanced phagocytosis of bacteria by alveolar macrophages was also inhibited by simultaneous addition of SP-A and LPS. Thus some biological activities attributed to either SP-A or LPS are inhibited in the simultaneous presence of the two agents. We therefore investigated the possibility of interaction and binding between SP-A and LPS molecules. Our biochemical data that include immunoblots and enzyme-linked immunosorbent assay support the notion that SP-A is capable of binding LPS, and this interaction is time and concentration dependent. The binding was partially inhibited (60%) by antibody to SP-A. The binding was calcium independent and was not affected by excess carbohydrates such as methyl alpha-D-mannopyranoside or heparin. Lipid A, the hydrophobic component of LPS, however, inhibited the SP-A-LPS interaction and also caused a partial reversal of the binding. Thus these results indicate that lipid A is associated with this binding. The biological implication of SP-A-LPS interaction, especially during inflammatory responses, is discussed.