PON1 (paraoxonase-1) detoxifies organophosphates by cleavage of active oxons before they have a chance to inhibit cholinesterases. The corresponding gene PON1 has common polymorphisms in both the promoter (-909, -162, -108) and the coding region (L55M, Q192R). The five PON1 genotypes were determined for maternal blood (n= 402) and cord blood (n= 229) as part of a study of the effects of organophosphate pesticide exposure on infant growth and neurodevelopment. PON1 enzymatic activities were determined for a majority of subjects. The population contained Caucasians, Caribbean Hispanics, and African Americans. PON1 activity was strongly dependent upon the promoter alleles in both maternal and cord blood. For example, PON1 activities for position -108CC, CT, and TT mothers were 146, 128, and 109 arylesterase U/mL (analysis of variance, p< 0.0001), whereas the same PON1 activities for the respective cord bloods were 49.0, 32.4, and 23.2 U/mL (p < 0.0001). Compared with adults, neonates had lower PON1 activity, implying reduced capacity to detoxify organophosphates. In addition there was a larger difference in activity between genotype groups in neonates than in adults. Because the five polymorphisms in PON1 occur in a short stretch of DNA, they were tested for linkage disequilibrium (LD). Significant LD was found among all three promoter polymorphisms as well as between promoter polymorphisms and L55M, with the strongest LD for Caucasians and the weakest for African Americans. The Caribbean Hispanics fall between these two groups. Surprisingly, significant LD also was observed between the promoter polymorphisms and C311S in PON2. LD between the promoter polymorphisms and Q192R was not significant.