A po B-100 of LDL can bind to both the LDL receptor and megalin, but the molecular interactions of apo B-100 with these 2 receptors are not completely understood. Naturally occurring mutant forms of apo B may be a source of valuable information on these interactions. Apo B-70.5 is uniquely useful because it contains the NH₂-terminal portion of apo B-100, that includes only one of the two putative LDL receptor–binding sites (site A). The lipoprotein containing apo B-70.5 (Lp B-70.5) was purified from apo B-100/apo B-70.5 heterozygotes by sequential ultracentrifugation combined with immunoaffinity chromatography. Cell culture experiments, ligand blot analysis, and in vivo studies all consistently showed that Lp B-70.5 is not recognized by the LDL receptor. The kidney was identified as a major organ in catabolism of Lp B-70.5 in New Zealand white rabbits. Autoradiographic analysis revealed that renal proximal tubular cells selectively removed Lp B-70.5. On ligand blotting of renal cortical membranes, Lp B-70.5 bound only to megalin. The ability of megalin to mediate cellular endocytosis of Lp B-70.5 was confirmed using retinoic acid/dibutyryl cAMP–treated F9 cells. This study suggests that the putative LDL receptor–binding site A on apo B-100 might not by itself be a functional binding domain and that the apo B–binding sites recognized by the LDL receptor and by megalin may be different. Moreover, megalin may play an important role in renal catabolism of apo B truncations, including apo B-70.5.