Five men isolated at Plateau Station, Antarctica, during the course of a south polar winter exhibited a significant drop (P<0.002) in the number of circulating leukocytes throughout the period of isolation. Shortly after return to normal social intercourse their white blood cell counts rose to normal levels similar to those observed prior to the isolation period. The total white blood cell count fell from a mean of 7,800/cu mm to a mean of 3,500/cu mm during the isolation period and returned to 7,500/ cu mm after return to civilization. The decrease was restricted to neutrophils. Although there was a significant relative lymphocytosis during the leukopenic period, the absolute number of lymphocytes did not deviate from normal values. This neutropenia may reflect an antigen deprivation consequent to the isolation from normal social contacts.