Aspartylglucosaminuria (AGU) is caused by deficient activity of the enzyme aspartylglucosaminidase (AGA). The structural gene for AGA has been assigned to the region 4q21-qter of chromosome 4. We have studied the map position of the AGU locus in relation to other marker loci on the long arm of chromosome 4 using linkage analyses. Restriction fragment length polymorphism alleles for the ADH2, ADH3, EGF, FG alpha and FG beta loci and blood group antigens for the MNS locus were determined in a panel of 12 Finnish AGU families. The heterozygous family members were identified by reduced activity of AGA in lymphocytes. Linkage studies were performed using both pairwise and multipoint analyses. Loose linkage of the AGU locus to the FG and MNS loci was observed (z = 1.16, z = 1.39, respectively). Multipoint analysis to the fixed map [ADH-(0.03)-EGF-(0.35)-FG-(0.11)-MNS] suggests that the location of the AGU locus is 0.05-0.30 recombination units distal to MNS (z = 3.03). The order cen-ADH-EGF-FG-MNS-AGU is 35 times more likely than the next best order cen-ADH-EGF-AGU-FG-MNS.