Abstract We have previously shown the binding of lowdensity lipoprotein (LDL) to immobilized rat C-reactive protein (CRP) and the formation of a fluid-phase complex between these two proteins. In this report we used immunoelectrophoresis and agarose gel electrophoresis to show increased anodic migration of the LDL particle as a result of the modification of LDL by rat CRP. The degradation of the modified 125 I-LDL by rat peritoneal macrophages was increased more than twofold in the presence of rat CRP. The increase in rat CRP-mediated 125 I-LDL degradation by macrophages was dependent on the concentrations of 125 I-LDL and rat CRP. This increased 125 I-LDL degradation was inhibited by phosphorylcholine. In contrast, the degradation of 125 I-acetyl-LDL by macrophages was not affected by rat CRP, although acetylated LDL inhibited the rat CRP-stimulated degradation of 125 I-LDL. Increasing concentrations of LDL did not affect the degradation of rat !25 I-CRP by the macrophages, which suggested that the rat CRP and the modified LDL did not enter the cell as a complex.