The study of immunosuppression after thermal injury has aroused great interest. The present study is an investigation of changes of plasma ir-beta EP in patients and their effect on lymphocyte responses to PHA following thermal injury, in order to establish a relationship between the immune and neuroendocrine system. Plasma ir-beta EP in eighteen burn patients was determined using the radioimmunoassay. At the same time, autoplasma and the autoplasma treated with anti-beta EP serum had been both tested for their effect on lymphocyte responses to PHA. The levels of plasma ir-beta EP were found to be elevated significantly in every phases during the first three days postburn (P < 0.0005-P < 0.001), and then declined to the normal level on the fourth or fifth day postburn. The lymphocytes cultured with autoplasma showed their inhibited responses to PHA during the first three days postburn and returned to normal on the fourth or fifth day postburn, while the autoplasma treated with anti-beta EP serum had an effect to improve the lymphocyte responses to PHA. The findings suggest that burn stress is a strong stimulation which may elevate plasma ir-beta EP and which in turn decrease the lymphocyte response to PHA. We assume that increased plasma ir-beta EP may be one of the main causes of immunosuppression after thermal injury.