Paraoxon is the bioactive metabolite of the organophosphate pesticide parathion. Desulphuration of parathion by liver enzymes or sunlight results in the formation of paraoxon which inhibits acetylcholine esterase (AChE) activity. In the present study, we analyzed the effect of a 6-week, subchronic treatment with two different daily intraperitoneal doses (30 or 40 nmol) of paraoxon on the immune system of BALB/c mice. At a dose of 30 nmol/day, body weight of treated animals was unchanged compared to the controls. In contrast, the higher dose (40 nmol/day) induced a reduction in body growth, particularly in the first 3 weeks of treatment, peaking at week 2 when the saline group showed a 14.2-fold increase in body weight gain compared to paraoxon-treated animals. Moreover, mice treated with either dose of paraoxon had a >50% reduction in AChE activity during the first 3 weeks of treatment, but by the end of the treatment (week 6), AChE activity returned to normal. With regard to immunological parameters, there was no significant difference in either total spleen weight or in the ratios of various spleen cell populations between control and paraoxon-treated animals. Furthermore, no changes were observed in mitogen-induced cytokine secretion from splenocytes of paraoxon-treated mice. Finally, subchronic exposure to paraoxon did not alter mortality of mice exposed to a bacterial infection with Salmonella typhimurium. These data suggest that although subchronic exposure to paraoxon induced a transient inhibition in AChE activity, it had no demonstrable effect on the host immune system. Organophosphate(OP) compounds are extensively used as pesticides throughout the world. It has been estimated that 95% of fatal pesticide poisonings occur in developing countries each year, leading to around 220,000 deaths (1). A WHO task group estimated there may be one million serious, unintentional, pesticide poisonings each year and, on the basis of a survey of self-reported minor poisoning, estimated that there may be up to 25 million agricultural workers in the developing world suffering an episode of poisoning each year (2). Whilst the exact proportion of these poisonings that are due to OPs is unknown, it is believed to be substantial. In 1995 there were 15,300 cases of pesticide poisoning in China of which 91% were due to OPs (3), and in 1996 it was estimated that