The duration of postoperative impairment in cell-mediated immunity was assessed by repeated skin testing with seven delayed type common antigens in 15 patients undergoing major elective abdominal surgery compared to a similar testing regimen in 10 healthy volunteers. All were skin tested four times, with 72-hour intervals, and in the surgical patients the first test was applied 2 days before surgery, followed by tests on postoperative days 1, 4 and 7. The tests were read after 48 h. Postoperatively, the skin test area decreased on day 3 (p less than 0.01) and recurred to preoperative levels on day 9. In contrast, the skin test area in the volunteers increased from test to test (p less than 0.001) during the study, confirming a previous finding of a vaccination effect. These results suggest that the postoperative immunosuppression is maintained for about 6-9 days.