The linkage relationships of the serum arylesterase paraoxonase (PON) was examined in our Danish material of normal families and in Danish and English cystic fibrosis families. Highest lod scores were found between PON and cystic fibrosis. The combined lod score for this relationship was z = 2.69 at theta = 0.07 in males and theta = 0.00 in females. When scored in accordance with a tentative three allele model for PON, the score was z = 3.70 at the same theta values. Linkage studies for PON against 64 other polymorphic marker systems did not give any lod score above +1.3 and PON still remains chromosomally unassigned. By the present screening about 2/3 of the genome could tentatively be excluded as the region of PON and cystic fibrosis.