Human plasma gelsolin, a 93,000-dalton actin-binding protein binds to human plasma fibronectin. Qualitative data obtained from experiments employing quasi-elastic light scattering, sucrose gradient sedimentation, gel filtration chromatography, and fibronectin polymerization indicate that gelsolin and fibronectin form a complex in solution. Solid-phase binding studies show that both human plasma and rabbit macrophage gelsolin bind to immobilized fibronectin with a Kd of about 1 microM in a 1:1 complex. The ability of gelsolin to interact with actin was not affected by the presence of fibronectin. Fibronectin also increased the amount of gelsolin binding to fibrin clots. Binding of gelsolin to fibronectin may serve to localize plasma gelsolin in regions where fibronectin is deposited, such as inflammatory sites.