In this communication we report the genetic properties of an insertion/deletion polymorphism in the signal peptide of the human apolipoprotein B (apo b) gene. There are two alleles of the apo B signal peptide; one codes for a peptide 27 amino acids in length and the other a peptide only 24 amino acids in length. Using the polymerase chain reaction the difference of nine nucleotides between the two alleles is readily detectable after electrophoresis of the amplification products. The relative frequencies of the Ins and Del alleles are 0.655 and 0.345, respectively. The apo B signal peptide genotypes are transmitted in a manner consistent with an autosomal codominant mode of inheritance with two alleles.