OBJECTIVE To investigate the effects of hypoxia on T-lymphocyte expression of IL-2 messenger RNA (mRNA), after cell activation with phorbol ester and ionomycin. METHODS Prospective, controlled, cellular trial. METHODS University research laboratory. METHODS EL4.6.1 cells, a murine T-cell lymphoma line. METHODS Tissue culture media was deoxygenated and flushed continuously with 100% helium to maintain a PO2 of 30 to 40 torr (< 40 torr [< 5.3 kPa]), or flushed with 10% oxygen/90% helium to maintain a PO2 of 45 to 55 torr (> 45 torr [> 6.0 kPa]). The pH was maintained between 7.3 and 7.6. The media was inoculated with EL4 cells. Aliquots of cells were obtained at intervals and divided into two groups: an immediate group, stimulated immediately, and an overnight group that was returned to normal incubator conditions of 5% CO2/humidified room air for 18 hrs before stimulation. RESULTS Gas tension, pH, cell count, and viability were determined for each aliquot. Cells were stimulated with phorbol myristate acetate and ionomycin for 4 hrs, at which time levels of interleukin-2 (IL-2) messenger RNA (mRNA) and gamma actin mRNA were measured by solution hybridization and enzyme immunoassay. The results were expressed as IL-2 mRNA/gamma actin mRNA ratio, normalised to baseline room air values. Cell viability and housekeeping functions (gamma actin mRNA expression) were unaffected by hypoxia. Cells exposed to a PO2 of < 40 torr (< 5.3 kPa) demonstrated a dramatic reduction in IL-2 mRNA expression with increasing duration of hypoxia. These effects persisted after an 18-hr recovery period. There was no effect on IL-2 mRNA expression when cells were exposed to a PO2 of > 45 torr (> 6.0 kPa). CONCLUSIONS The regulation of IL-2 transcription in the T lymphocyte appears to be exquisitely sensitive to changes in oxygen tension. Exposure to a PO2 of < 40 torr (< 5.3 kPa) causes prolonged impairment of IL-2 mRNA expression. IL-2 is an important growth factor for T and NK cells, and plays a pivotal role in the regulation of the host's immune response. The long-lasting effects of brief hypoxic exposure may, in part, explain the critically ill patient's predisposition to infectious complications.