The phagocytic function (e.g., phagocytosis, chemiluminescence, and chemotaxis) of polymorphonuclear leukocytes (PMNs), delayed hypersensitivity, and serum opsonic capacity were studied in 29 patients who underwent major surgical procedures, primarily abdominal surgery. The phagocytic capacity and chemiluminescence were normal in all patients as compared with healthy donors, and no difference in phagocytosis was found before or after operation. However, PMNs of 14 patients (48%) showed diminished chemotactic activity after surgery, but the influence of surgery on chemotaxis could not be demonstrated. About 30% of the patients had depressed delayed hypersensitivity after operation, as measured by skin testing using three recall antigens. The mean induration of the skin tests was 6.9 +/- 3.4 mm before operation and 3.6 +/- 2.9 mm thereafter (P = 0.006). The opsonic capacity of patient sera was measured as uptake by normal donor PMNs of staphylococci opsonized in patient serum. Eleven of 19 preoperative serum samples had an opsonic capacity comparable to that of normal control sera; the opsonic capacity of postoperative serum samples of the same patients was significantly decreased as compared with preoperative values. In seven patients the opsonic capacity of postoperative serum samples correlated with decreased levels of IgG in the sera but not with complement activity.