In the current study the authors investigated the connection between the proliferation rate of stimulated hu- man peripheral blood lymphocytes with some other characteristics of the population: expression of immunological markers, spontaneous genome damage, radiosensitivity. The population's proliferation index (PI) was taken as a measure of the rate. It was calculated using the composition of a cell population, which was cyto- kinesis-blocked with a cytochalasin B. If the genotoxic action is absent, the PI does not correlate with the spontaneous frequency of cells with micronuclei or with cell radiosensitivity, but is tightly linked with immunological indexes. It has been determined that after stimulation the level of marker-positive cells (CD25, CD69 and Ki67) is closely related to PI and is greater in the populations with lower proliferation rates. Irradiation of a cell culture 48 h after stimulation at a dose of 1 Gy leads to a correlation between PI and radiosensitivity, measured directly after the irradiation and in the same time frame as the PI measured in the non-irradiated population. The irradiated population's PI is not connected with the level of marker expression.