Abetalipoproteinemia (ABL) is an autosomal recessive disease characterized by the inability of the liver and intestine to secrete apolipoprotein B (apoB). Mutations in the microsomal triglyceride transfer protein (MTP) gene, but not the apoB gene, are responsible for the ABL phenotype. It is not clear how loss of MTP in ABL patients leads to a complete, but specific, block in the secretion of apoB. It is to this question that our work is directed. In cultured cells lacking MTP, translocation of apoB is completely arrested, leading to the hypothesis that apoB requires MTP in order to completely enter the lumen of the endoplasmic reticulum, the site of lipoprotein assembly. We examined this hypothesis by determining the presence in plasma of distinct N-terminal apoB peptides, produced exclusively from translocation arrested apoB, in the plasma of six ABL patients and six normal subjects. The data show that N-terminal apoB peptides are present in the plasma of six ABL patients, whereas intact apoB-100 was barely detectable. Moreover, the plasma of all six ABL patients displayed a 2000-fold increase in the amount of an 85 kDa N-terminal apoB peptide relative to apoB-100. These data provide the first in vivo data supporting the essential role that MTP plays in apoB translocation. In normal humans, varied expression of MTP may be responsible for the post-transcriptional regulation of apoB secretion.