Naturally occurring mutant forms of apolipoprotein B (apoB)-100 may be able to provide valuable information on the structure-function relationships of apoB with the low density lipoprotein (LDL) receptor. ApoB-67, recently identified in a kindred displaying apoB levels 25% of normal (Welty et al. J. Clin. Invest. 1991. 87: 1748-1754), is predicted to contain 3040 amino acids and therefore, contains part of the epitope for antibody 4G3, which blocks binding of LDL to the LDL-receptor. To determine whether the amino terminal 67% of apoB-100 is important for binding to the LDL receptor, the apoB-67-containing lipoprotein particle was purified from plasma by gradient ultracentrifugation. The fractions containing apoB-67 were in the density range 1.049-1.070 g/ml. These fractions were pooled and adsorbed onto an affinity chromatography column containing the monoclonal antibody, MB-47. The epitope for MB-47 is two nonlinear domains between amino acids 3429 to 3453 and 3507 to 3523; therefore, apoB-100 will bind to the MB-47 column but apoB-67 will not. The resulting apoB-67-containing particles were completely devoid of apoB-100. In competitive binding studies, the apoB-67 lipoprotein particle did not compete with 125I-labeled apoB-100-containing LDL particles for binding, uptake, or degradation by normal human fibroblast monolayers. We conclude that the amino terminal 67% of apoB-100 in the naturally occurring lipoprotein particle does not appear to contain a functionally relevant epitope of the LDL-receptor binding domain.