The pulsating bubble technique was used to study the surface activity of binary phospholipid mixtures containing dipalmitoyl-phosphatidylcholine (DPPC) and an unsaturated acidic phospholipid such as egg phosphatidylglycerol (egg PG), 1-palmitoyl-2-oleoyl-PG (POPG) or egg phosphatidic acid (egg PA) in the presence of surfactant-associated protein B (SP-B) and calcium. The relative surface activities were DPPC/egg PG/SP-B (7:3:1%) greater than DPPC/POPG/SP-B (7:3:1%) greater than DPPC/egg PA/SP-B (7:3:1%). The Wilhelmy surface plate technique was utilized to investigate the interaction between pure SP-B in the bulk phase (0.9% NaCl/1.5 mM CaCl2) and preformed DPPC or phosphatidylglycerol (PG) monolayers. Although SP-B injected into the bulk phase reduces the surface tension of a clean surface, no evidence was obtained for the insertion of SP-B into surface monolayers at equilibrium surface tension. Surface radioactivity measurements and the Wilhelmy surface plate technique were also used to study the potential interactions between liposomes of DPPC/POPG (7:3) with or without SP-B and surface monolayers of [14C]DPPC or [14C]POPG. No exchange of phosphatidylcholine (PC) or PG was found between the monolayer and liposomes. We also compared the adsorption of pure POPG or 1-palmitoyl-2-oleoyl- phosphatidylcholine (POPC) and binary mixed liposomes with DPPC in the presence or absence of SP-B and calcium. DPPC/POPG/SP-B (7:3:1%) was found to be more surface active than pure POPG plus 1% SP-B in the presence of calcium. Injection of SP-B into the bulk phase promoted the adsorption of DPPC/POPG liposomes to a greater extent than POPG liposomes. The enhanced adsorption was dependent on the presence of calcium. In contrast to PG, DPPC/POPC/SP-B (7:3:1%) was less surface active than pure POPC plus 1% SP-B either in the presence or absence of calcium. Our findings suggested that the molecular composition and organization of mixed monolayers play an important role in the surface activity of the surfactant.