Monoclonal antibodies reacting with specific determinants on lymphocytes and their subpopulations were used to study the influence of surgery under general or epidural anaesthesia on the number of T-cell (T11), T-suppressor cells (T8), and T-helper cells (T4). The T-cell fraction decreased significantly during surgery, but was only moderately reduced on the first postoperative day. The change in the T-cell fraction was the same under both types of anaesthesia. The helper and suppressor cell fractions were not significantly altered at any time during the study, and again there was no difference between the two anaesthetic regimens. The ratio of helper cells to suppressor cells (the T4/T8 ratio), which is widely used as an indicator of immunosuppression, showed no significant variations during the study, even though there were individual variations. Changes in the T4/T8 ratio were not correlated to the amount of blood transfusions, nor was there any correlation between the T4/T8 ratio and the thymidine uptake in mitogen-(PHA)-stimulated lymphocytes. T-lymphocytes are depressed during and after surgery under general anaesthesia. From the present study, this does not seem to be due to changes in the balance between helper- and suppressor T-cells.