Human serum paraoxonase is a polymorphic enzyme that is capable of catalyzing the hydrolysis of paraoxon and other organophosphates, some carbamates and certain aromatic carboxylic acid esters. This enzyme is specified by two allelic genes at one autosomal locus (isozymes ‘A’ and ‘B’). The purpose of this study was to examine the paraoxonase activity of 105 Turkish subjects. Paraoxonase activities ranged from 39.6 to 278.2 nmol p-nitrophenol formed per ml of serum per min. Paraoxonase phenotypes could be clearly identified by salt and paraoxonase:arylesterase activity ratio characteristics. The gene frequencies were 0.632 for the low activity allele (A) and 0.368 for the high activity allele (B).