1. Long term alteration of immune functions caused by low doses of nerve agent sarin were studied in rats exposed to sarin by inhalation. The alteration of immune functions by sarin was monitored by using two methods (the evaluation of in vitro spontaneous as well as stimulated proliferation of spleen cells and in vitro bactericidal activity of peritoneal macrophages) at 3, 6 and 12 months following sarin exposure. 2. The results indicate that not only symptomatic but also asymptomatic dose of sarin is able to alter some immune functions at six and twelve months following exposure to sarin. 3. Thus, not only organophosphorus insecticides but also nerve agents such as sarin can be potentially immunotoxic even at very low doses that do not cause clinically manifested intoxication following the inhalation exposure. The ability of sarin at low doses to alter immune functions seems to be really long term (up to 12 months following the exposure).