Lipid A isolated from lipopolysaccharide of Yersinia pseudotuberculosis was used for immunization of rabbits to afford antisera to lipid A with titers of 1: 640 in the passive hemolysis test. Exhaustion of immune serum with sheep erythrocytes decreased antibody titers up to 1:160. Authentic samples of 2-(DI-3-hydroxytetradecanoyl)amino-2-deoxy-D-glucose 6-phosphate, 2- tetradecanoylamino-2-deoxy-D-glucose 6-phosphate and 2-acetamido-2-deoxy-D-glucose 6-phosphate have been synthesized in order to carry out a comparative study of inhibitory activity of these com- pounds and lipid A using a system of lipid A and antiserum to lipid A. As a result, the immunodominant moiety of the lipid A of Y. pseudotuberculosis proved to contain a D-glucosamine residue acylated with 3-hydroxytetradecanoic acid at the amino group. The nature of the fatty acid acylating the amino group of glucosamine does not play an important role in the structure of immunodominant moiety of lipid A. [ABSTRACT FROM AUTHOR]