Fibronectin is associated with cell attachment and migration and interacts with fibrin, collagen and glycosaminoglycans; thus, it may be a factor in the focal proliferation of smooth muscle cells and collagen in atherosclerosis. We have measured, by rocket immunoelectrophoresis, the concentrations of soluble and collagenase-releasable fibronectin in normal human aortic intima and different types of atherosclerotic lesions. Soluble fibronectin concentration showed no significant difference between normal intima and lesions, but was 6-8-times higher than expected on the basis of plasma concentration and molecular mass. The concentration free in the interstitial fluid was about 3-times the expected level, suggesting that it originates from local synthesis as well as plasma insudation. In tissue, about half the fibronectin appeared to be reversibly associated with tissue components. Incubation with collagenase released fibronectin equal to twice the soluble fraction from normal intima and early proliferative lesions. In more advanced plaques that were accumulating lipid, the amount released was significantly higher (P less than 0.05) and more than 3-times the soluble fraction, suggesting that it might be involved in lipid accumulation. However, there was no correlation between release of fibronectin and bound low-density lipoprotein.