Disruptions of the Treg differentiation and functioning processes can play one of the crucial roles in the pathogenesis of radiation-induced malignant neoplasms in residents of the Techa riverside villages, who were chronically exposed in the low-to-medium dose range with predominant damage to the red bone marrow (RBM). This study aimed to determine the effect of radiation exposure, gender, age at the time of examination, and ethnicity on concentration of FOXP3 protein in lysates of mitogen-stimulated peripheral blood mononuclear cells in chronically exposed individuals in the period of cancer effects development. The main group consisted of 30 people aged 67–80 years, predominantly female and Turks. The comparison group included 10 unexposed individuals of similar age, gender, and ethnicity. In the main group, the mean dose to RBM was 867 mGr, to the thymus and peripheral lymphoid organs — 125 mGr. After 24-hour in vitro PHA stimulation, mononuclears were lysed, and the concentrations of the total protein and FOXP3 (using quantitative enzyme immunoassay) were measured. Among the different dose groups, there were no significant differences in FOXP3 concentration in mitogen-stimulated mononuclears (prior to the stimulation: 0 pg/ml in the comparison group and 3.50 ± 1.50 (0–27.19) pg/ml in the main group at p = 0.349; after the stimulation, respectively: 1.54 ± 1.51 (0–15.16) pg/ml and 9.71 ± 3.86 (0–77.92) pg/ml, p = 0.512). The variability of individual values is slightly higher in the main group than in the comparison group. Preliminary results allow concluding that the dose to RBM, thymus and peripheral lymphoid organs, age at the time of examination, gender, and ethnicity have no statistically significant effect on the concentration of FOXP3 protein in the lysates of the mitogen-stimulated peripheral blood mononuclear cells of chronically exposed people.