We studied the changes in peripheral lymphocyte subsets, mitogen responsiveness, natural killer (NK) cell activity, and interleukin-2 (IL-2) production in patients with or without diabetes receiving coronary artery bypass surgery. Group I (GI): 9 diabetic patients comprising three on oral diabetics during therapy, two on insulin therapy, and four on alimentary therapy. Group II (GII): 12 non-diabetic patients (borderline diabetics excluded). age, amount of blood transfusion, number of grafts, aortic cross-clamp time (ACC), cardio-pulmonary bypass time (CPB), and operative time (OP) did not significantly differ between the groups. Lymphocyte subsets were measured using monoclonal antibodies and IL-2 production was measured by radio-immuno assay using IL-2 labeled with I125. All variables were measured the day before, the day after, 3 days after and 7 days after the operation. The number of lymphocytes and their subsets (CD3+, CD+, CD8+, 4/8 ratio, IL-2R+) did not significantly differ between the groups, but in GI patients, the number of OKIa1 positive lymphocytes were significantly lower than in GII the day before and 7 days after the operation. II-2 production on the day after the operation was significantly (p < 0.05) reduced from the preoperative level in both groups. On 3 days, there was a significant difference (p < 0.05) between the two groups: IL-2 production in GI (3.1 +/- 2.6 U) was remarkably lower than in GII (6.6 +/- 4.0 U). IL-2 production in GII was significantly correlated to the number of CD4 positive lymphocytes, but this was not true in GI. Mitogen responsiveness to stimulation with PHA was not significantly different between the groups. NK cell activity on the first postoperative day was significantly reduced (p < 0.01) in the both groups, but there was no difference between the groups. The % change in IL-2 production (%IL-2) in GII on 3 days after the operation was significantly correlated to the amount of blood transfusion (r = -0.7, p = 0.0077) but that in GI was not. %IL-2 was not significantly correlated to ACC, CPB, OP, or age in both groups. This study clearly showed that diabetics who underwent coronary artery bypass surgery suffered depression of cellular immunity, in particular, IL-2 production, which might be a key factor in cellular immunity. It showed a decrease in helper T lymphocyte function after surgery, implying postoperative immunodeficiency in diabetics.