A method for identifying two human serum paraoxonase phenotypes in vitro has been developed based upon the effect of NaCl upon paraoxonase activity. In a sample population of 336 individuals from the United States, 53.9% of the samples had serum paraoxonase that was highly stimulated (60%-257% above the control activity) by 1 M NaCl (salt-responsive), whereas the activity of the remaining samples was not salt-responsive (-23%-35%). The degree of stimulation was consistent and reproducible in frozen samples collected from an individual over a two-year period. Pedigree studies with 37 families indicate that the salt-responsive characteristic is inherited as a simple autosomal, Mendelian trait. Although the salt-responsive individuals on the average had a higher level of activity when assayed without added salt (basal activity) than did the non-salt-responsive individuals, there was considerable overlap in the basal paraoxonase activities. The quantitative polymorphism in serum paraoxonase activity observed in other laboratories is associated with a qualitative difference, quite possibly due to two distinct isozymic forms of the enzyme. A new designation for these alleles is proposed, and some preliminary studies on the molecular basis of the polymorphism are reported.