Neutrophil function and the prevalence of infections in 85 workers occupationally exposed to phosphoorganic pesticides were examined and compared to those of age- and sex-matched healthy controls. Activity of cholinesterase (ChE) in red blood cells (RBC) and in serum was used as marker of organophosphate toxicity. A decrease of CHE activity was observed in all groups of workers, both in serum and in RBC, and was dependent on the degree of exposure to the pesticides. A marked impairment of neutrophil chemotaxis stimulated with zymosan-activated serum (from workers or controls) was observed in all groups of workers. There was no significant change of polymorphonuclear neutrophil (PMN) migration after their preincubation in 10% autologous serum or washed cells. However, neutrophil chemotaxis was normal after stimulation with specific mononuclear-derived chemotactic factor (MDCF). The area of random migration increased in those working directly in the production of organophosphates. A decrease of adhesion and an increase of nitroblue tetrazolium-dye reduction was also found. The frequency of upper respiratory tract infection (in 68.8%) was greater in workers than in controls and similarly to their recurrence was dependent upon the time of exposure to the insecticides. A normal frequency of infections localized outside of the upper respiratory tract was observed. It may be concluded that (1) impairment of chemotaxis in the workers is not dependent on serum factors; (2) stimulation of neutrophil chemotaxis by MDCF occurs by a different mechanism to their stimulation by zymosan-activated serum; (3) the increase of morbidity in the workers is probably caused by prolonged inhalation of both the insecticides and solvents; (4) the impaired function of neutrophils may be not so important as expected. However, the local effects of the pesticides should be explored.