INFASURF ADSORBS RAPIDLY TO THE SURFACE OF THE AIR:LIQUID INTERFACE AND MODIFIES SURFACE TENSION SIMILARLY TO NATURAL LUNG SURFACTANT. A MINIMUM SURFACE TENSION OF = 3 MN/M IS PRODUCED IN VITRO BY INFASURF AS MEASURED ON A PULSATING BUBBLE SURFACTOMETER. EX VIVO, INFASURF RESTORES THE PRESSURE VOLUME MECHANICS AND COMPLIANCE OF SURFACTANT-DEFICIENT RAT LUNGS. IN VIVO, INFASURF IMPROVES LUNG COMPLIANCE, RESPIRATORY GAS EXCHANGE, AND SURVIVAL IN PRETERM LAMBS WITH PROFOUND SURFACTANT DEFICIENCY.