PURPOSE: Neonatal respiratory distress syndrome (RDS) is caused by the deficiency of pulmonary surfactants in the newborn. We aim to determine and compare the physical and biological activities of the three surfactants currently on the market, namely Newfactan(R), Surfacten(R) and Exosurf(R). METHODS: For physical activities, we performed the stable microbubble test (SMR) and Pulsating Bubble Surfactometer (PBS). The minimum and maximum surface tensions measured at 1 and 5 minutes allowed us to create the surface-tension diagrams, from which the compressibility at a surface tension of 10 mN/m was calculated for all three products. The biological activities were compared using the pressure-volume curves measured from premature rabbit fetuses. RESULTS: For all three products, the concentration of surfactant and the number of stable microbubbles exhibited a proportional increase in relationship. For both Newfactan(R) and Surfacten(R), the minimal surface tensions were lower than 10 mN/m at 1 and 5 minutes. Hysteresis was evident at 1 and 5 minutes for both Newfactan(R) and Surfacten(R), and their surface tensions were reduced below 10 mN/m at 20% surface compression. As for Exosurf(R), all the hysteresis measurements were below expectations, and the reduction in surface tension during compression was also minimal. The compressibilities at a surface tension of 10 mN/m and measured at 1 and 5 minutes for Newfactan(R) and Surfacten(R), were less than 0.020 cm/dyne at almost concentrations. The pulmonary surface areas of the fetal rabbits were, after aeration by a maximum of 30 cmH2O, adequately maintained after decompression to 5 cmH2O in the groups treated with Newfactan(R) and Surfacten(R). However, the lung volume was not maintained upon decompression to 5 cmH2O in the groups treated with Exosurf(R). CONCLUSIONS: Newfactan(R) and Surfacten(R) were effective agents in prevention of pulmonary collapse in premature lungs.