Double-label immunofluorescence microscopy and immunoelectron microscopy were performed on stationary cultures of Nil 8 fibroblasts to determine if fibronectin and basement membrane heparan sulfate proteoglycans play coordinated roles in cell-to-substrate adhesion. Relationships between subcellular matrix fibers containing fibronectin plus proteoglycan, and focal contacts associated with microfilament bundles, were studied simultaneously using interference reflection microscopy, differential interference contrast microscopy, and immunofluorescence microscopy. Cells maintained in 0.3% FBS were doubly stained with monospecific anti-fibronectin IgG and antibodies against a basement membrane proteoglycan purified from the EHS (Engelbreth-Holm-Swarm) tumor. Coincident patterns of fibronectin and proteoglycan-containing fibers were found to codistribute with focal contacts and microfilament bundles in both early (6-h) and late (24-h) cultures. The early cells showed doubly-stained fibers colinear with substrate adhesion sites in 43% of the sample, while 100% of the later cells exhibited these coaligned matrix-cytoskeletal attachment complexes. Immunoelectron microscopy showed that both of these antigens were situated in the same type of extracellular matrix fiber that appeared to be loosely associated with the cell surface membrane. We hypothesize that the appearance of proteoglycan in subcellular matrix fibers of these fibroblasts might stabilize fibronectin-containing cell-to-substrate contacts.