W e have previously described a disorder, normotriglyceridemic abetalipoproteinemia, that is characterized by the virtual absence of plasma low density lipoproteins and complete absence of apoB-100, but with apparently normal secretion of triglyceride-rich lipoproteins containing apoB-48. The patient's plasma lipoproteins were shown on polyacrylamide gels and by antibody mapping to have a new truncated apoB variant, apoB-50, circulating along with her apoB-48. We have found this individual to be homozygous for a single C-to-T nucleotide substitution at apoB codon 2252, which produces a premature in-frame stop codon. Thus, this is a rare example of homozygous hypobetalipoproteinemia. Electron photomicrographs revealed that the diameters of particles in the d less than 1.006 g/ml lipoprotein fraction, in both the postprandial and postabsorptive state, are bimodally distributed. The molar ratio of apoE to apoB in these particles is 3.5:1, similar to normal VLDL. The plasma LDL interval contains both spherical and cuboidal particles. Autologous reinfusion of labeled d less than 1.006 g/ml lipoproteins showed exponential disappearance from plasma, with an apparent half-removal time of 50 min, somewhat slower than for normal chylomicrons but within the normal range for VLDL. The calculated production rate for apoB was within the normal range in this subject. A very small amount of label was found briefly in the IDL fraction, but none at any time in LDL or HDL. Therefore, because LDL particles that contain apoB-50 lack the putative ligand domain of the LDL receptor, we conclude that the very low level of LDL is due to the rapid removal of the abnormal VLDL particles before their conversion to LDL can take place.