An estimated 4 to 5 million persons work in agriculture; within this sector, mixers, loaders, applicators, and other workers who directly handle agricultural chemicals are at highest risk for pesticide-related illnesses. Among field workers, seasonal workers engaged in certain cultivation and harvesting operations are also at risk of exposure to pesticide residues. Biological screening for agricultural workers is limited at present to pesticide mixers, loaders, and applicators, and consists almost exclusively of measurements of cholinesterase activity to estimate inhibition by organophosphate and carbamate compounds. We review here the measurement and interpretation of cholinesterase activity in occupationally exposed populations and the correlation between cholinesterase activity and symptoms of acute and chronic illness among agricultural workers. The only state to mandate medical surveillance for agricultural workers is California: criteria for surveillance, baseline cholinesterase determination, periodicity of testing, and criteria for removal from and return to work are described, and alternative approaches for screening and clinical evaluations are discussed.