Radioimmunoassay of pulmonary surface-active material in the tracheal fluid of the fetal lamb.

We measured pulmonary surfactant antigen by radioimmunoassay and phospholipid by lipid phosphorus assay in fluid collected during 12-hour periods from the trachea of lambs in utero between 102 and 147 days of gestation. Both phospholipid and antigen were detectable at 114 days and were present at low concentrations until 137 days. The concentrations then increased rapidly and reached 5- to 10-fold higher concentrations before parturition. The ratio of lipid phosphorus to antigen increased approximately 5-fold as the lungs passed from canalicular to alveolar architecture and approximated the adult ratio only during the alveolar stage of development.