The apolipoprotein A-IV (apoA-IV) allele frequencies were determined in 387 adult Finns by immunoblotting after isoelectric focusing of serum. The gene frequencies were: A-IV1 = 0.942 and A-IV2 = 0.058. The phenotypes of 147 mother-child pairs studied were in accordance with the two allelic modes of inheritance. In 2 subjects, a rare apoA-IV variant was found.