In this study we have examined 532 migrant farm workers engaged mainly in the cultivation of vegetable crops, in both greenhouses and openfarms, and an equal number of controls. Erythrocyte acetylcholinesterase activity (AChE) was measured to determine the degree of toxicity due to exposure to organophosphate and carbamate pesticides in the farm workers employed either as foremen (41.5%) or farmers (58.5%). The mean ages of the farm workers and controls were 35.2 +/- 7.4 (mean +/- SD) years and 34.6 +/- 7.1 years. AChE activity of the farm workers and controls was 3.89 +/- 0.64 UI/ml (mean +/- SD) and 4.15 +/- 0.29 UI/ml. The haemoglobin adjusted erythrocyte cholinesterase activity (HAChE) was 29.96 +/- 4.14 (mean +/- SD) for farm workers and 32.10 +/- 2.26 for controls. AChE activity was very highly significantly lower for the foremen (3.76 +/- 0.69) compared to farmers (3.98 +/- 0.59) (Student's t-test = 4.13, p = 0.0001). HAChE was also very highly significantly lower for foremen (29.24 +/- 4.37) compared to farmers (30.46 +/- 3.88) (Student's t-test = 3.64, p = 0.0001). The poorly controlled used of pesticides in the farms appeared to have caused sub-clinical intoxication in the farm workers and indicated the need for training and implementation of hygiene practices.