Fibronectin, a major pericellular glycoprotein of adherent cells, was predominantly present in fibrillar structures in human fibroblast cultures as shown by indirect immunofluorescence. In conventional "patching experiments" where one day old cells were exposed to anti-fibronectin IgG in the cold, washed, and reincubated at 37 degrees no redistribution was seen. However, continuous exposure of the cultures to IgG at 37 degrees resulted in redistribution. The fibrillar structures were lost and fibronectin aggregates (patches) were found. Fab-fragments had no such effect. These results support the findings that fibronectin is predominantly a matrix protein and show that matrix components may be redistributed in cell culture conditions.