The organophosphate cholinesterase inhibitor paraoxon is hydrolysed by serum paraoxonase/arylesterase. A genetic polymorphism of paraoxonase (PON) activity which determines high versus low paraoxon hydrolysis in human populations, may determine sensitivity to parathion poisoning. We demonstrate that arginine at position 192 specifies high activity PON whereas a glutamine specifies the low activity variant. Allele-specific probes or restriction enzyme analysis of amplified DNA allow for the genotyping of individuals. PON maps to chromosome 7q21-22, proximal to the cystic fibrosis gene, in agreement with previous genetic linkage studies.