We have examined the binding of native and cell-modified low density lipoprotein (LDL) to gels of Type I collagen. Diffusion of native 125I-LDL into the collagen gel was slow, reaching equilibrium after 24 to 48 hours, while L-3H-glucose, a low molecular weight marker, equilibrated in 6 hours. Binding of 125I-LDL was measured at 48 hours as the amount associated with the collagen after extensive washing. Binding was saturable with an increasing concentration of LDL. Prior incubation with cell-free culture medium resulted in modest, but progressive, increases in electrophoretic mobility and binding to collagen. Incubation with cells produced a marked increase in electrophoretic mobility and a 5- to 10-fold increase in collagen binding; the presence of butylated hydroxytoluene during incubation prevented both effects. These changes in LDL were induced by porcine aortic endothelial cells, smooth muscle cells, human skin fibroblasts, and a variety of cell lines, as well as by acetylation. There was a curvilinear relationship between the amount of LDL protein bound and the net negative charge of the LDL; increasing net charge was associated with progressively greater increases in binding. These results suggest a potential role for collagen in trapping lipid in the extracellular matrix of arterial intima by slowing the diffusion of and by binding LDL. The data also demonstrate that binding of LDL to collagen is enhanced by modifications that increase its net negative charge.