A semiquantitative tintometric field kit has been used in the developing world for almost 30 years to measure whole blood cholinesterase levels in persons exposed to organophosphate pesticides. The validity of this screening kit was evaluated among 79 workers heavily exposed to organophosphates by comparison with a reference assay for erythrocyte cholinesterase. Overall correlation between the two methods was good. However, either sensitivity or specificity of the tintometric kit was less than 75% for each of the three tintometric categories commonly used to define the limit of normal. Because baseline erythrocyte cholinesterase levels were not available for this population, the true sensitivity and specificity of the tintometric assay may be even lower.