Effects on monocyte-mediated cytolysis and thymidine uptake in PHA-stimulated lymphocytes were studied with cells from healthy donors and sera from patients undergoing hip replacement under epidural- or general anaesthesia. Sera were sampled before induction of either epidural- or general anaesthesia (I), 15 min after induction of anaesthesia (II), during surgery 60-90 min after start of induction of anaesthesia (III), and the next morning at 8 o'clock (IV). Serum sample II from patients operated under general anaesthesia significantly depressed both monocyte cytolysis and thymidine uptake in lymphocytes. At sampling time III and IV the monocyte cytolysis was insignificantly reduced. There was no suppression of lymphocyte-thymidine uptake at these sampling times. The sera from patients operated under epidural anaesthesia did not influence the thymidine uptake in lymphocytes. However, sera drawn from epidural anaesthesia patients at sampling time IV significantly depressed the monocyte cytolysis (the sample was drawn the next morning 3-4 h after discontinuation of the epidural anaesthesia). We conclude that the depressive effect of surgery under general anaesthesia on monocyte cytolysis and lymphocyte proliferation is partly transferable by serum factors. The depressive effects on monocytes and lymphocytes of sera drawn 15 min after induction of general anaesthesia may be due to the effect of the induction of thiopentone.