After surgery blood lymphocyte levels fell to one-third of the pre-operative value. Since this depression was transient, and followed the peak of serum cortisol closely, it was probably due to a redistribution of lymphocytes from the blood to the tissues. The proportion of activated lymphocytes, as measured by the incorporation of 3H-thymidine in vitro without added mitogen, was substantially increased about 5-8 days after operation. The greatest number of S-phase lymphocytes was found after operation combined with blood transfusion but surgery alone and blood transfusion alone each produced significant increases. The proportions of B lymphocytes, T helper cells and T suppressor cells remained nearly constant despite the fluctuations in total lymphocyte counts. The response of lymphocytes in vitro to phytohaemagglutin (PHA) was doubled on average 7 days after surgery provided the test was performed in the patient's own serum. In pooled serum there was no consistent change in the response to PHA. It is argued that none of the observed changes necessarily signify that a phase of immunosuppression follows surgical operations.