This report describes the results of a long‐term monitoring study of 36 grape harvesters who were occupationally exposed to organophosphate pesticide residues following reentry into vineyards during September and October 1976. The study was designed to evaluate biochemical parameters associated with organophosphate pesticide exposure, to relate the occurrence of changes in these parameters to environmental residues of cholinesterase‐inhibiting pesticides, and to evaluate techniques for monitoring the health of agricultural workers routinely exposed to organophosphate pesticide residues.