To determine whether culturing peripheral blood mononuclear cells at atmospheric oxygen levels skews responses in comparison with culturing lymphocytes at physiologic oxygen levels, we cultured peripheral blood mononuclear cells at 5%, 10%, and atmospheric (20%) gas-phase oxygen for 5 days. We found that incubator oxygen levels influenced lymphocyte proliferation stimulated by two commonly used stimuli: Con A and antibodies that crosslink surface CD3 and CD28 to mimic antigen presentation. In both cases, proliferation increased as gas-phase oxygen levels increased. In contrast, oxygen levels did not influence proliferation stimulated by phytohemagglutinin, another commonly used mitogen. Similarly, oxygen levels did not impact cell viability in unstimulated cultures. Thus, we conclude that the influence of oxygen levels on proliferation depends on the stimulus, and, most importantly from the standpoint of immune responses, culturing cells at atmospheric rather than physiologic oxygen levels results in significantly increased proliferation responses to the CD3͞CD28 crosslinking, a proliferation stimulus commonly used to mimic T cell antigen receptor signaling.CD3͞CD28 ͉ incubator ͉ T cell ͉ tissue culture P roliferation of lymphocytes in response to antigen or mitogen stimuli is a common measure of ex vivo lymphocyte function. Virtually all of our ideas about the autoimmune and protective mechanisms in which human lymphocytes participate are derived from studies based on this methodology. However, the CO 2 incubators in which nearly all of these studies have been conducted largely contain air to which a small percentage of CO 2 has been added. Thus, it is perhaps surprising that nearly all of what has been concluded to date about human lymphocyte function is based on evidence from cell-culture studies conducted at atmospheric oxygen levels (20% oxygen) that are well above the levels of oxygen to which cells are exposed in the human body.Determining the oxygen levels available to cells in the body is a difficult task. Blood has been shown to have a partial pressure of oxygen (pO 2 ) of 80-100 mmHg (1 mmHg ϭ 133 Pa) (1), which is equal to 10-12.5% O 2 . pO 2 levels in healthy tissue are considered to be in the range of 30-50 mmHg (2), which is equal to 3-6% O 2 . Thus, we estimate that standard culture conditions (20% oxygen) expose cells to Ϸ2-to 5-fold higher concentrations of oxygen than they would likely encounter in vivo.The disparity between the in vivo pO 2 and the pO 2 in incubators maintained at atmospheric oxygen levels clearly raises the question of whether current culture conditions are appropriate for the study of lymphocyte and other cell functions. In fact, several studies have already examined the effect of incubator oxygen levels on lymphocyte functions such as proliferation (3), cytolytic activity (4), cytokine production (4, 5), and antibody secretion (6). However, because the methods used and the findings obtained in these studies vary considerably, no clear picture has yet emerged as to whether ...