We conducted studies dealing with the synthesis and release of fibronectin in vitro by polymorphonuclear leucocytes (PMNL). The specific purpose of our study was to look for any changes in these events as they happen in inflammatory vs non-inflammatory PMNL. We used PMNL isolated from the synovial fluid of patients with rheumatoid arthritis as a source of inflammatory cells and PMNL isolated from peripheral blood as a source of non-inflammatory cells. Marked differences were observed. Using 35S-methionine metabolic labelling and SDS-polyacrylamide gel analysis, we were first able to clearly observe an increased synthesis of fibronectin by inflammatory PMNL when compared to non-inflammatory PMNL. Furthermore, the release of fibronectin in vitro by these cells was increased by factors of up to 20 when compared to non-inflammatory peripheral blood PMNL. Experimental evidence was also obtained which strongly suggests that fibronectin exists in a stored form inside the inflammatory PMNL we used in this study. Finally, we observed that PMNL are capable of synthesizing a 95 kD gelatin binding protein which appears to be distinct from fibronectin.