Monocyte functions have been investigated in 18 patients undergoing total hip arthroplasty performed under either general anaesthesia or epidural anaesthesia. Serum cortisol increased significantly both per- and postoperatively in the general anaesthesia group compared to minor changes in the epidural group. The spreading of monocytes on plastic surfaces possibly reflects the phagocytic capacity of the cells. This spreading index of monocytes derived from patients operated under epidural anaesthesia and cultured in medium containing autologous serum showed a significant increase per- and postoperatively compared to the spreading index of monocytes from patients operated under general anaesthesia. This difference increased with increasing culture time. The ability of monocytes to induce lysis in malignant cells was reduced to 45% peroperatively in monocytes derived from patients operated under general anaesthesia and cultured in medium containing autologous serum. In the epidural group the lysis was nearly unchanged from the preoperative value, making the difference between the groups highly significant (P less than 0.01). On the first postoperative day, the difference between the groups was less pronounced. The differences in spreading index and cytolytic capacity were not seen when the monocytes were cultured in medium with pooled AB-serum, thus indicating a serum factor responsible for the monocyte depression when the patients were operated under general anaesthesia.