The titers of antibodies to the common allergenically active fraction of enterobacterial antigens in clinically healthy males aged 20-35 years, constantly living at the moderate (Kazan) and high (Norilsk) latitudes, were studied. The most essential changes in the character of the titer distribution were revealed in the populations of persons who had been constant residents of the Arctic regions for 5-10 years. This was manifested by an increase in the number of sera with excessively high (exceeding 1:5120) and low (less than or equal to 1:20) titers. The average indices of the reactivity to intestinal automicroflora were significantly higher in the residents of the Arctic regions than in the corresponding control group. In persons living in the North for still longer periods, as well as in those who lived at high latitudes since their birth, such shifts in the reactivity to enterobacteria were less pronounced.