The extensive international use oforganophosphorus compounds (OP) results in numerous acute intoxications each year. OPs inhibit acetylcholinesterase, the enzyme responsible for breaking down the neurotransmitter acetylcholine. The World Health Organization recognizes cholinesterase (ChE) biomonitoring as a preventive measure against OP overexposure. The aim of this study was to determine if dermal OP contamination could interfere with current field ChE biomonitoring assays, which use a fingerstick blood sample. In this study we also sought to determine if high levels of a plasma enzyme, A-esterase, could protect ChE from inhibition by hydrolyzing environmentally generated oxons potentially present in a fingerstick sample. A heparinized venous blood sample was collected from a volunteer. Erythrocyte acetylcholinesterase (AChE) and plasma butyrylcholinesterase (PChE) activities were measured using a field-based colorimetric cholinesterase kit. ChE dose-response curves were constructed by allowing 10-ul blood samples to contact environmentally realistic levels of OP thioate and oxon for 10 s. An inhibition threshold could not be established for PChE when exposed to oxon within the time necessary to perform a fingerstick analysis. AChE was also inhibited by trace amounts of oxon consistent with previously reported environmental levels. These findings suggest that the reliability of field-based biomonitoring results is limited if OP residues remain on a skin surface at the time of sample collection. A-esterase's role in protecting ChE activity was investigated using capillary and venous blood from 30 unexposed individuals. Baseline ChE activities were measured, as were individual A-esterase activities using paraoxon, diazoxon, and phenyl-acetate as substrates. Results were then compared to ChE activities measured after 10 s of contact with an environmentally realistic amount of Of, containing 1 % oxon. Both ChE activities were significantly inhibited, with capillary values being significantly more inhibited than their venous counterparts. ,However, no protective effect could be associated between the degree of A-esterase activity and the subsequent level of ChE inhibition observed in an individual's blood. These results suggest that ( I ) if there is any uncertainty about O f skin contamination, venous blood would be a more appropriate specimen to employ when using field ChE biomonitoring kits-it is collected in larger volumes and has essentially no direct contact to dermal surfaces; and (2) A-esterase activity demonstrates no protective effect against ChE inhibition upon a blood droplet's brief contact with an O f residue containing traces of oxon.