The immunodepressive effect of surgical procedures is well known. Immunotherapy with thymic hormones has used a long time to prevent surgical immunitary damage. In cardiac surgery, we can observe a hard immunitary depression caused by the cardiopulmonary bypass (CBP). In this paper we evaluate the response to immunotherapy, with an extractive thymic hormone: thymostimulin at the dosage of 1 mg/kg/day; 20 patients of a group of 40 underwent CBP. Immunological monitoring was obtained with the "multitest CMI", a disposable skin test to evaluate changes of lymphocytes, especially T cells. Results shows a significant increase of response in patients treated with thymostimulin versus control. A larger sample and a deeper immunitary assessment will be provided in the future.