Despite their broad clinical use, there is no standardized comparative study on the functional, biochemical, and morphologic differences of the various commercial surfactants in relation to native surfactant. We investigated these parameters in Alveofact, Curosurf, Exosurf, and Survanta, and compared them with native bovine (NBS) and porcine (NPS) surfactant. For Curosurf and Alveofact the concentrations necessary for minimal surface tensions < 5 mN/m were six to 12 times higher (1.5 and 3 mg/ml, respectively) than with NPS and NBS. Exosurf and Survanta only reached 22 and 8 mN/m, respectively. Increasing calcium to nonphysiologic concentrations artificially improved the function of Alveofact and Curosurf, but it had little effect on Exosurf and Survanta. Impaired surface activity of commercial versus native surfactants corresponded with their lack in surfactant protein SP-A and decreased SP-B/C. The higher surface activity of Curosurf compared with Alveofact corresponded with its higher concentration of dipalmitoylphosphatidylcholine (DPPC). Despite their enrichment in DPPC Survanta and Exosurf exhibited poor surface activity because of low or absent SP-B/C. Ultrastructurally, Curosurf and Alveofact consisted mainly of lamellar and vesicular structures, which were also present in NPS and NBS. Exosurf contained crystalline structures only, whereas the DPPC-enriched Survanta contained separate lamellar/vesicular and crystalline structures. We conclude that in vitro surface activity of commercial surfactants is impaired compared with native surfactants at physiologic calcium concentrations. In the presence of SP-B/C, surface activity corresponds to the concentration of DPPC. Our data underscore the importance of a standardized protocol at physiologic calcium concentrations for the in vitro assessment of commercial surfactants.