The study included 40 healthy male agricultural workers (age range 24-29 years) with a mean age of 26.3 years who were exposed to the pesticides malathion and chlorpyrifos with varied durations of exposure (2-5 years) in Jordan. Thirty unexposed healthy males of the same communities (age range 23-28 years) with a mean age of 26.10 years who had no occupational contact with pesticides, were used as a control group. Twenty of the 40 workers and 16 of the 30 controls were smokers. All the participants in the study completed a questionnaire about their medical and occupational history. Any of the individuals who had been exposed to any agent known to interfere with the results, such as exposure to X-ray, to viral infection, or to drugs within a period of 3-4 months preceding the blood sampling was excluded.

Peripheral blood samples were collected by the classical method and sent coded to the Genetics Laboratory (Department of Biological Sciences, University of Jordan, Amman, Jordan) for cytogenetic analysis. Lymphocyte cultures were performed according to Hungerford [24], with minor modifications. The lymphocytes were cultured in RPMI 1640, supplemented with 20% fetal calf serum, penicillin-streptomycin solution and 0.2 mL phytohemagglutinin. Incubation was at 37°C for 48 hours and, during the last 2 hours, colcemid (0.1 mg/mlwas added to produce mitotic arrest. The cells were then treated with hypotonic potassium chloride (0.075 M KCI) and fixed with glacial acetic acid in methanol (1:3). Air-dried slides were stained with 10% Giemsa. A total of 200 cells per individual were examined. Only cells with complete chromosome number were scored for chromosomal aberrations which were classified and recorded as recommended by the World Health Organization [25]. The data obtained were statistically analyzed by means of the t-test.