Acetylcholinesterase inhibition was determined for 666 Kenyan agricultural workers; 390 (58.6%) mainly pesticide applicators exposed to organophosphate and carbamate pesticides and 276 (41.4%) unexposed controls from four rural agricultural areas during 1993 and 1994. Baseline levels were depressed in the exposed group (6.1 +/- 0.84; 4.09 +/- 0.84) but not in the unexposed group (5.83 +/- 0.91; 5.60 +/- 0.87). Acetylcholinesterase inhibition was found in all exposed individuals and led, on average, to a decrease of baseline acetylcholinesterase levels of 33% (+/-12%). The control groups had a nonsignificant decrease of only 4% (+/- 8%). The exposed subjects in Naivasha (flower growers) had the largest inhibition (36%), followed by Homabay (cotton growers) (35%) and Wundanyi (vegetable growers) (33%). Those in Migori (tobacco growers) had, by far, the least inhibition of acetylcholinesterase activity (26%), indicating inherent factors that led to less inhibition. Acetylcholinesterase activity levels of 115 exposed individuals (29.6%) and no controls were depressed to values below 60% of baseline levels. The dramatic inhibition observed could lead to chronic clinical and subclinical intoxication. These findings show that acetylcholinesterase inhibition can be used as an indicator of organophosphate and carbamate poisoning in occupationally exposed agricultural workers. There is, therefore, an urgent need for primary prevention programs to monitor and address occupational exposures to these hazardous substances in agriculture in Kenya and other developing countries, as well as to use integrated pest management strategies in crop protection.