When testing the proliferative activity of 14 strains of permafrost microorganisms in the reaction of blast transformation of human lymphocytes in vitro, a strain (Alcaligenes sp.) with mitogen properties was isolated (20-fold increase in the rate of lymphocyte proliferation in comparison with the control). Four strains activated lymphocyte proliferation by 3-9 times in comparison with the control. Three strains produced substances with cytostatic properties and reduced proliferation activity by 33-43% and one strain (Bacillus sp.) almost completely suppressed phytohemagglutinin-induced lymphocyte proliferation. These data indicate that strains with a unique immunobiological potential are concentrated in the population of permafrost microorganisms that have undergone rigorous evolutionary selection.