We have studied the effects of human thrombin on the isolated pericellular matrix of cultured human lung fibroblasts. Cell-free matrices were prepared from confluent cultures of cells by extraction with sodium deoxycholate and hypotonic buffer after radiolabeling the cultures with [14C]glycine. After the extraction, only a few radiolabeled polypeptides were retained on the culture dishes. These were identified as fibronectin, procollagens, and as yet unidentified polypeptides with molecular weights of 180 000, 140 000, 66 000 and 43 000. The matrices were exposed to thrombin in serum-free medium and the changes in the matrix-associated proteins were studied in autoradiograms of polyacrylamide gels. As a result of the treatment, there was massive release of both fibronectin and procollagen from the matrices into the medium. In addition , thrombin cleaved the 66 000 dalton polypeptide to a 62 000 dalton form that remained in the matrix. Collagenase treatment did not bring about the release of fibronectin or affect the 66 000 dalton protein. Some procollagen was also cleaved by thrombin; when high concentrations of thrombin were used cleavage of fibronectin took place. These effects of thrombin may operate in wounded areas in vivo.