Although most subjects with familial defective apolipoprotein B-100 (FDB) have raised plasma low-density lipoprotein (LDL) levels, a few have LDL levels within the normal range. We have previously identified two normocholesterolemic FDB heterozygotes in an affected family. Results obtained from a study of this family are compatible with a major genetic contribution to the normocholesterolemia in the two heterozygotes. However, our findings are not compatible with inheritance of a variant normal allele at the apolipoprotein B locus in this family that neutralizes the effect of an FDB allele on the plasma LDL level. Polymorphic variations at the apolipoprotein E and LDL receptor loci did not explain the presence of normal LDL levels in the two heterozygous FDB subjects.