The purpose of this study was to: review the measurement and interpretation of cholinesterase activity in populations occupationally exposed to pesticides; review the correlation between cholinesterase activity and symptoms of acute and chronic illness; and discuss biologic monitoring of exposed workers for intact pesticides and their metabolites. Current biological monitoring practices were summarized and thoughts were presented on potential developments in biological surveillance programs for agricultural workers. Intraindividual and interindividual variation, age, sex, race, disease, genetic variance, and pregnancy were discussed. The following laboratory methods of cholinesterase determination were evaluated comparatively: electrometric, colorimetric, titrimetric, tintometric, and field spectrophotometric. In addition to the acute symptoms of cholinergic excess, constellations of nonspecific central nervous system symptoms are often seen as sequelae to cases of acute poisoning, and in association with chronic exposures to low levels of organophosphates. When these nonspecific symptoms were investigated in farm worker studies, group correlations were found between prevalence of symptoms and the moderate levels of cholinesterase inhibition encountered. Neither the presence nor the severity of these symptoms were well correlated with cholinesterase inhibition for individuals. This suggests a major limitation in use of cholinesterase screening to predict health effects of chronic low level residue exposure. The authors suggest that biological monitoring programs for agricultural workers exposed to organophosphate pesticides should include the following components: identification of high risk populations requiring surveillance; baseline cholinesterase determinations; periodic surveillance; and criteria for removal from work.