Y.pseudotuberculosis thermostable lethal toxin as shown to affect the functional activity of human blood leukocytes. Still neutrophils and monocytes exhibited different reaction to the introduction of the toxin. In monocytes, the toxin in a dose of 0.2 microgram/4 x 10(4) cells inhibited the formation of active forms of oxygen (AFO) and activated the production of superoxide dismutase (SOD). In neutrophils, the toxin affected only the activity of glutathione reductase, inducing a decrease in its activity. The toxin, depending on its dose, stimulated a rise in the level of cyclic adenosine monophosphate (cAMP) in mononuclear cells, but inhibited it in neutrophils. When the toxin was introduced together with indomethacin (10(-7) M), the level of cAMP increased in mononuclears and decreased in neutrophils in comparison with the action of the toxin, introduced alone. Indomethacin prevented the action of the toxin on mononuclears with respect to AFO, SOD and cAMP, which was indicative of the mediating role of prostaglandins in the mechanism of the action of the toxin.