[en] Total sublethal irradiation with x-rays of the rabbits immunized with the Salmonella TH 901 antigen induces a decrease in the serum antibody level as compared with nonirradiated controls. Administration of the polypeptide thymic (TP) extract to rabbits immunized with antigen and x-rayed in similar conditions produces a stimulation of antibody formation in these animals as compared to the nontreated controls. The level of antibodies is altered in the animals irradiated, and treatment with the TP extract shows that it has a protective effect on the organism