A mixture of synthetic phospholipids containing 90% Dipalmitoylphosphatidylcholine and 10% Dipalmitoylphosphatidylglycerol was tested for its suitability as surfactant substitute. Different methods of preparation were employed. The substance was studied by chromatography, Wilhelmy balance, and in rabbits after tracheal instillation during mechanical ventilation. When prepared as suspension of multilamellar liposomes in NaCl, the phospholipid samples displayed the essential surfactant features of adsorption from the subphase, spreading to a monolayer and lowering the surface tension to 3.7 dyn/cm at compression. For a dose of 2.55 microgram/cm2, the stability index was 1.83 +/- 0.29. Samples prepared with ultrasound were not surface-active. Sonication also destroyed the surface-tension-lowering ability of previously active samples. No signs of local toxicity were found in the treated animals. When radioactive DPPC/DPPG was instillated into the trachea of the ventilated rabbit, autohistoradiography demonstrated radioactive material in the alveoli covering the alveolar wall with a thin layer. Provided that proper techniques of preparation and administration are employed, the phospholipids tested are a promising surfactant substitute worthy of clinical study.