The National Institute for Occupational Safety and Health (NIOSH) recommends monitoring blood cholinesterase levels among workers who handle pesticides as a means of detecting exposure to organophosphate compounds that inhibit these enzymes. Traditionally this medical surveillance involves collecting venipuncture blood samples that are sent to a laboratory for analysis. Because this can be a time consuming process, and because workers often object to serial venipuncture sampling, rapid assay technology is being developed. To test the reliability of an on site finger stick system, we tested 97 workers for cholinesterase inhibition using the standard venipuncture method and using a spectrophotometric field kit. We also assessed exposure by analyzing urine samples for para-nitrophenol p-NP. The workers, who were cleaning private residences contaminated with the organophosphate methyl parathion, were tested before starting work and then repeatedly over a 7-week period. We compared results of the two methods to determine the kit's ability to accurately predict cholinesterase levels in humans. There were no instances of pesticide exposure detected by urinalysis for p-NP. N either venipuncture erythrocyte cholinesterase mean 10,991 international units IU/L, coefficient of variation 14.5% nor test kit erythrocyte cholinesterase mean 3.4 IU/ml, coefficient of variation 14.8% levels identified workers with 30 percent or greater depression from their personal baseline. Assuming that the laboratory method is a gold standard, the specificity of the finger-stick test kit for our study is 100 percent. Sensitivity could not be estimated. Real time cholinesterase monitoring will enhance pesticide surveillance. Our study design provided a useful way of assessing existing technologies. However, the reliability of these technologies can be evaluated only if quantifiable human exposure actually occurs during the study period.