Sepsis remains a major risk in the high mortality and morbidity after surgery for obstructive jaundice. The reasons for the increased susceptibility to infection are unknown. This study examined interleukin 2 (IL-2) production and the lymphocyte response to PHA mitogen in 31 patients with obstructive jaundice. Among them, 18 patients were simultaneously investigated by enumeration of T lymphocyte subsets in peripheral blood with APAAP technique. The results showed that the patients had significantly decreased IL-2 production and lymphocyte response to PHA mitogen. The percentage of Leu 3a (helper/inducer T cell) in the patients was significantly lower than that in normal controls. Leu 3a/Leu 2a (suppressor/cytotoxic T cell) ratio was significantly lower in these patients. The reduction of IL-2 production correlated significantly with the suppression of lymphocyte proliferation but not with the percentage of Leu 3a cells. From these results, it may be suggested that the reduction of IL-2 production in the patients with obstructive jaundice is an important reason for the suppression of T lymphocyte proliferative response, not merely a reflection of the decrease of helper T cells.