We investigated the effects on immune function after progressive hypobaric hypoxia simulating an ascent to 25,000 ft (7620 m) over 4 weeks. Multiple simultaneous in vitro and in vivo immunologic variables were obtained from subjects at sea level, 7500 ft (2286 m), and 25,000 ft during a decompression chamber exposure. Phytohemagglutinin-stimulated thymidine uptake and protein synthesis in mononuclear cells were reduced at extreme altitudes. Mononuclear-cell subset analysis by flow cytometry disclosed an increase in monocytes without changes in B cells or T-cell subsets. Plasma IgM and IgA but not IgG levels were increased at altitudes, whereas pokeweed mitogen-stimulated in vitro IgG, IgA, and IgM secretion was unchanged. During exposure to 25,000 ft, in vitro phytohemagglutinin-stimulated interferon production and natural killer-cell cytotoxicity did not change statistically, but larger intersubject differences occurred. IgA and lysozyme levels (nasal wash) and serum antibodies to nuclear antigens were not influenced by altitude exposure. These results suggest that T-cell activation is blunted during exposure to severe hypoxemia, whereas B-cell function and mucosal immunity are not. Although the mechanism of altered in vitro immune responsiveness after exposure to various environmental stressors has not been elucidated in humans, hypoxia may induce alterations in immune regulation as suggested by in vitro immune assays of effector-cell function.