To study the influence of low-level sarin inhalation exposure on immune functions, inbred BALB/c mice were exposed to two low concentrations of sarin for 60 minutes in the inhalation chamber and then infected with Francisella tularensis LVS on the 7th day following the exposure to sarin. 24 hours after infection, the level of some isotypes of antibodies (IgM, IgA) against tularaemia was significantly decreased regardless of the sarin concentration used while the lymphoproliferation was significantly increased regardless of the mitogen and sarin concentration used. Later, the level of some isotypes of antibodies (IgM, IgA) against tularaemia and the vitality of Francisella tularensis LVS was significantly increased in the case of exposure of mice to clinically symptomatic concentration of sarin (7 days after infection) while the lymphoproliferation was significantly decreased regardless of the concentration of sarin when specific tularaemic antigen Ag4 was used as a mitogen (3 weeks after infection). Thus, the results indicate that not only symptomatic but also asymptomatic dose of sarin is able to alter the host resistance and reaction of immune system, especially at 24 hours and 7 days following infection with Francisella tularensis LVS. Nevertheless, the alteration of immune functions following the inhalation exposure to a symptomatic concentration of sarin seems to be more pronounced.