The blastogenic responses of lymphocytes to PHA-stimulation and lymphocyte lymphokine production have been studied in 27 patients undergoing total hip replacement, operated under either general anaesthesia or epidural anaesthesia. The lymphocytes were isolated from the patients the day before operation, during the operation and on the first postoperative day. The assays were carried out with lymphocytes cultured in medium containing 25% of the patient's serum (Au-medium) and in medium containing 25% of pooled AB+ serum (AB-medium). In the epidural anaesthesia group, the blastogenic response to PHA of lymphocytes incubated in Au-medium increased per- and postoperatively to 132% (n.s.) and 155% (P less than 0.05) compared to preoperative values. In the general anaesthesia group, the PHA-response decreased to 63.7% peroperatively (P less than 0.05) and 88.7% postoperatively (n.s.). When the lymphocytes were incubated in AB-medium, the PHA-response increased in both groups. Lymphocyte lymphokine production in the epidural group was unchanged both per- and postoperatively in both media. However, in the general anaesthesia group there was a reduction in lymphokine activity both per- and postoperatively. This reduction was most pronounced peroperatively when the lymphocytes were incubated in Au-medium (64.2%, P less than 0.05). Control lymphocytes incubated in sera from general anaesthetized patients resulted in significantly lower lymphokine activity (P less than 0.05) in comparison with cultures with sera from patients in the epidural group. These findings indicate a lymphocyte depressing factor in serum from patients operated under general anaesthesia, which is absent during operations in epidural anaesthesia.