Studied was the effect of the preparations phenitrothion (Agria-1050) and phosalon (Agria-1060) on the immunologic reactivity of sheep through following up the phenomena of agglutination and phagocytosis at the injection of a Salm. gallinarum antigen. An unknown property of these preparations was established, consisting in the stimulation of some defense immunobiologic processes in the body of animals, at their application in minimum doses per os, simultaneously with the injection of a bacterial antigen. Phenitrothion applied orally to sheep at the rate of 50 mg per kg body mass daily, seven times in the course of sixty days, with the injection of a suspension of a killed Salmonella gallinarum culture (containing 10(7) microbial cells per 1 cm3-3 cm3 i/v, and 5 cm3 four days later, s/c per animal) stimulated agglutinin production, increasing it from 9 to 17 times; it raised the phagocytic number by 2.5 to 3 times, and the phagocytic index--by 1.7 to 2.14 times. With control animals the increase was 3-4, 1.4-1.5, and 1.10-1.19 times, respectively. Phosalon applied to sheep in following the same pattern at the rate of 13 mg per kg body mass led to a rise of the agglutinintiter from 22 to 26 times; the phagocytic number was raised by 2.6 to 4.3 times, and the phagocytic index--by 1.6-2.3 times.