Immunological aspects of the adaptation process were investigated in 57 male test subjects that stayed for 30 days at an altitude of 3600 m above sea level (Eastern Pamir Mountain Range). The uneventful development of adaptation was accompanied by a short-term decrease in the number and activity of T-lymphocytes. An acute mountain disease led to a distinct deficiency of T-cell immunity which still persisted on test day 30. Besides, the content of zero cells in circulating blood was increased and the blast-transformation reaction of lymphocytes to concanavalin A was inhibited. Prior to the ascent the test subjects who were susceptible to the acute mountain disease showed a lower content of T-lymphocytes and a higher content of zero cells in circulating blood.