Sapropel was tested for its effect on the permeability of erythrocytic membranes and on the erythrocytic and serum levels of ceruloplasmin, malonic dialdehyde, and dienic conjugates in rats after acute poisoning by carbofos, a malathion insecticide. The increased processes of free radical oxidation during carbofos poisoning were suggested by the higher rates of chemiluminescence of erythrocytes and peripheral blood serum. No significant changes in the parameters of blood oxidative stress in the carbofos-poisoned animals during therapeutic-and-prophylactic use of sapropel indicate that the latter has antioxidative properties. Application of sapropel to normal animals exerted no effect of the studied parameters of oxidative stress. In acute carbofos poisoining, oral sapropel produced a pronounced antioxidative effect, which opens up new vistas for its practical application.