The surface distribution of the scavenger receptors for acetylated low-density lipoprotein (acetyl-LDL) and their endocytic behavior were studied by the direct immunoperoxidase method using monomeric conjugates of horseradish peroxidase with Fab' antibody raised against LDL. The receptors were demonstrated to be distributed diffusely on the surface membrane of cultured peritoneal macrophages, with preferential localization in coated pit regions. With temperature shift from 4 degrees C to 37 degrees C, acetyl-LDL bound to the surface membrane rapidly disappeared, but became detectable in coated vesicles or lysosomes. Further incubation in the presence of acetyl-LDL revealed lipid vacuoles devoid of a limiting membrane in the cytoplasm, transforming macrophages into typical foam cells. These data suggest that the binding of acetyl-LDL to its receptors triggers the clustering of the receptors into the coated pit regions through which acetyl-LDL is endocytosed by coated vesicles to be degraded in lysosomes with subsequent intracellular accumulation of cholesterol esters.