Multiple parameters of in vivo and in vitro immune function were measured serially before, during, and following nephrectomy in 12 normal renal transplant donors. All in vitro functions studied (total blood lymphocyte count, B-cell count, T-cell count, mitogen blastogenic response, and mixed leukocyte reactivity as both responder and stimulator) decreased on induction of anethesia and continued to fall during and after operation to reach a low point on the evening after nephrectomy. Depth of depression and rate of recovery varied with the individual function, but all were near normal by the fifth postoperative day. The in vivo delayed hypersensitivity response to cutaneously administered recall antigens declined more gradually and was still falling at the fifth postoperative day. Return of preoperative skin response was delayed, being complete for streptokinase/streptodornase (SK/SD) by 10 to 14 days but incomplete as long as 2 to 3 weeks for mumps and Candida antigens. Serum immunoglobulins did not change. These findings suggest incomplete correlation among the responses to the commonly used in vitro assays of cellular immunity and poor correlation with the in vivo tests. Although surgery and anesthesia results in measurable depression of immune response, clinically significant problems did not arise in these patients.