Endotracheal administration of human or xenogenic surfactant preparations is an effective treatment of the respiratory distress syndrome of preterm infants. The application of large amounts of phospholipids to the lung may result in a significant alteration of the local immune response. We studied the influence of the bovine surfactant preparation SF-RI 1 (Alveofact) on lymphocyte functions in vitro. PHA-induced cell proliferation and immunoglobulin synthesis in the presence of whole surfactant as well as six different defined phospholipids were investigated. A marked concentration-dependent suppression of immunoglobulin production independent of the immunoglobulin isotype and cell proliferation was observed in the range of 5 ng/ml-3 mg/ml of a single phospholipid (or SF-RI 1 respectively). It could be demonstrated that suppression of lymphocyte functions was only due to the phospholipid content of the surfactant preparation. These data indicate that in vivo immune functions may be significantly altered by the administration of exogenous surfactant. This may be particularly important in the presence of primary or secondary pulmonary infections.