Interaction of native and thrombin-modified human low density lipoproteins (LDL) with immobilized homologous fibronectin (either covalently bound to Sepharose or adsorbed from blood serum on collagen-Sepharose) was studied. Treatment of LDL with thrombin at pH 7.5 and 37 degrees within 60 min (thrombin/apo B ratio 1:20 w/w) led to formation in LDL preparations of 3 new fragments of apoprotein B which were detected by polyacrylamide gel electrophoresis in presence of sodium dodecylsulfate. Chromatography of native and thrombinmodified LDL on fibronectin-Sepharose showed that 30% of the modified LDL and 2% of native LDL were bound to fibronectin-Sepharose at physiological pH values and NaCl concentrations. Study of the interaction of LDL with fibronectin adsorbed on collagen-Sepharose showed that thrombin-treated LDL partially released fibronectin from the sorbent due to the formation of a modified LDL-fibronectin complex. Native LDL did not act in a similar manner. Complexes of modified, LDL with fibronectin were detected under conditions of both electrophoresis in 3% polyacrylamide gel and immunoelectrophoresis. Interaction of LDL with fibronectin may promote accumulation of lipoproteins in the vascular wall and thus may serve as a model system for evaluation of the extent of atherogeneity of LDL and detection of the modified LDL in vivo.