Background and Aim: Organophosphate compoounds are cholinesterase activity inhibitors. Contact with them cause changes in the blood antioxidant enzyme levels and lipid peroxidation. The aim of this research was to investigate the simultaneous changes in cholinesterase, total serum antioxidant levels, and activities of some of the erythrocyte antioxidant enzymes and find possible correlations among these variables in pesticide factory workers. Materials and Methods: This research was a cohort study carried out, during 2006, on the employees of a pesticide factory in Golestan Province, Iran. Blood samples were taken at the beginning (n=81 subjects) and after 3 months (n=63, of the initial 81). Erythrocyte glutathione peroxidase (GPx), superoxide dismutase (SOD) and total antioxidant levels were determined enzymatically (Randox, UK) and serum SOD was measured by the ELISA method (IBL, Germany). For the determination of malondialdehyde (MDA) a chemical method (based on thiobarbituric acid) was used. Confidence interval was 95% for all statistical tests.Results: Levels of serum and erythrocyte SOD, serum MDA, and both serum and eryhthrocyte GPx significantly increased (paired t-test, p<0.05), while the serum total antioxidant level decreased (paired t-test, p<0.05). No statistically significnt correlation was seen between cholinesterase and changes in the other variables. Conclusion: Contact with organophosphorus compounds will cause increases in lipid peroxidation and decreases in total antioxidants. It is suggested that, while more severe physical protective measures are taken, nutritional supplements be taken by the personnel to increase protection against hazardous materials.