A serial determination of pulmonary surfactant apoprotein-A (SP-A) was made on tracheal aspirates from seven intubated infants with different types of respiratory failure in the first week of life. A two-site immunoassay with monoclonal antibodies was adopted to determine the SP-A concentration. The concentrations of albumin in the same samples were also assayed, and these data were expressed as the ratio of SP-A to albumin (SP-A/albumin ratio), and evaluated against clinical data such as the arterial-alveolar oxygen tension ratio (a/APO2) or ventilatory index. In infants with respiratory distress syndrome, the SP-A/albumin ratio was initially low, and increased gradually in the first few days of life with the improvement of a/APO2 and ventilatory index. The complication of pulmonary hemorrhage due to patent ductus arteriosus (PDA) resulted in a temporary decrease in the ratio. The infant with transient tachypnea of the newborn showed higher concentration from the first day of life and, in the course of PDA without pulmonary hemorrhage, the ratio did not decrease. The cases of congenital pneumonia showed the SP-A/albumin ratio remaining low while the infection was evident. These data suggest that the SP-A/albumin ratio of the tracheal aspirate can be used for the quantitative and qualitative evaluation of endogenous pulmonary surfactant in newborn infants with different respiratory disorders.