To study the persistence of Y. pestis capsular antigen, or fraction 1 (F1), in the body of less important plague carriers in the Mountain Altai and Transbaikal natural foci, as well as in experimentally infected ticks, the liquid-phase competitive radioimmunoassay (RIA) was used for the first time. In this study RIA showed, due to its sensitivity, doubtless advantages over traditional methods, such as the passive hemagglutination (PHA) test and the antibody neutralization (AN) test, and made it possible to detect F1 in picogram amounts. RIA revealed that F1 persisted in Siberian long-tailed gophers for up 14 months after the infection of the animals in diffusion chambers and for 7 months after their infection by subcutaneous injection. Experiments on Daurian pikas confirmed that, in comparison with the PHA and AN tests, RIA ensured fourfold effectiveness in the detection of antigen F1. The study of infected mites revealed that antigen F1 could be retained in them for more than a year and detected by RIA techniques in 10% of cases. The data obtained in this investigation indicate that the persistence of microorganisms should be studied mainly with the use of new-generation tests, and RIA, being one of the most sensitive techniques, deserves wide approval and introduction into the practical work of institutions intended for plague control.