Results of serological examination of 81,853 mammals from the Muyunkum and 26,303 from the Eastern Kyzylkum deserts were compared with results of a corresponding bacteriological examination for plague. In both foci and in all phases throughout the epizootic of plague, the serological method made it possible to detect infected animals 6-10 times more frequently in comparison with the bacteriological method. The big meriones mice, i.e., the basic carriers of plague in natural foci, were found to be infected considerably more (in percentage) frequently than the meridian meriones. Differences in the reactivity of meriones to the invasion of plague microbe were observed in both foci (the indices of titres in the antigen neutralization reaction were considerably higher in the Eastern Kyzylkums). The serological method of investigation is highly informative in evaluating both the present state of epizootic activity of a plague focus and its activity in the past.