Incorporation of pulmonary surfactant into fibrin inhibits its plasmic degradation. In the present study we investigated the influence of the surfactant proteins (SP)-A, SP-B and SP-C on the fibrinolysis-inhibitory capacity of surfactant phospholipids. Plasmin-induced fibrinolysis was quantified by means of a ¹²⁵I-fibrin plate assay and surfactant incorporation into polymerizing fibrin was analysed by measuring the incorporation of ³H-labeled DPPC into the insoluble clot-material.Incorporation of a calf lung surfactant extract(Alveofact®) and of an organic extract of natural rabbit large surfactant aggregates (LSA) into a fibrin clot revealed a stronger inhibitory effect on plasmic cleavage of this clot than a synthetic phospholipid mixture (PLX) and unprocessed LSA. Reconstitution of PLX with SP-B and SP-C increased, whereas reconstitution with SP-A decreased the fibrinolysis-inhibitory capacity of the phospholipids. The SP-B effect was paralleled by an increased incorporation of phospholipids into fibrin. We conclude that the inhibitory effect of surfactant incorporation into polymerizing fibrin on its susceptibility to plasmic cleavage is enhanced by SP-B and SP-C, but reduced by SP-A. In case of SP-B increased phospholipid-incorporation may underly this finding.