Normal rat kidney cells were cultured in medium supplemented with normal fetal bovine serum (FBS) or FBS depleted of fibronectin. The cell surface fibronectin of these cultures was visualized by indirect immunofluorescence using species-specific antisera for either rat fibronectin or bovine fibronectin. Anti-rat-fibronectin revealed fibrillar structures on the cells grown in either normal medium or fibronectin-depleted medium. Anti-bovine fibronectin revealed similar fibrillar networks, but only on the cells grown in medium containing bovine fibronectin. Staining in each case was abolished by absorption with the homologous antigen. It appears that exogenous fibronectin was incorporated into the same structures as endogenous fibronectin. This finding suggests that circulating fibronectin may serve as a building block for the assembly of extracellular matrix, possibly by cells which are incapable of synthesizing it.