An automated assay for plasma cholinesterase (CHE) activity (EC 3.1.1.8) was used to screen 44 farmworkers engaged in fruit crop spraying in the Somerset West, CP, area. Seven had markedly reduced CHE activity, at least 30% below the lower limit of the urban reference range, while the group as a whole had moderately depressed levels. A screen of 14 nursery-workers from the same farm showed a similar picture with the 1 individual engaged in spraying duties having an extremely low CHE activity of 1,595 U/I (lower normal limit for adult males is 4,687 U/I). Clinical complaints were few and were described as 'asthma' and 'chronic bronchitis' by the nursing sister in charge of the staff clinic. The mean body mass of the 7 farmworkers with substantially reduced CHE activities was 7,4 kg lower than the others and this was accompanied by significantly diminished body mass index. Removal of the 7 worst affected individuals from spraying duties for a period of 6 weeks resulted in a statistically significant increase in CHE activity of 1,700 U/I, whereas the values in the remaining workers remained unchanged. These results strongly suggest that at least 17,5% of male rural workers engaged in crop spraying suffer from chronic organosphosphate poisoning. The somewhat depressed levels of CHE activity in the rural group as a whole may also reflect chronic exposure of a lesser degree. All 7 female workers tested had enzyme activities within the normal range.