Field methods are needed to detect and monitor the organophosphate pesticide exposure of young children. Twenty children, aged 11 to 18 mo, living in an agricultural community along the United States/Mexico border were enrolled in a pilot study investigating methods to detect pesticide exposure. Healthy children were recruited at pediatric clinics with the informed consent of their parents. Venous blood samples were collected from children twice, 4 wk apart. Cholinesterase activity was compared in whole heparinized venous blood with venous blood samples dried on filter paper. Although the amount of activity in the dried blood was consistently less than in the heparinized blood, the activity was significantly correlated: Spearman r = .6 (p = 0.01). This dried blood method may be used during field studies to evaluate changes in cholinesterase values in children over time.