Total lymphocyte count, lymphocyte cell-surface markers (OKT3, OKT4, OKT8, and B-1), serum complement factors (C3 and C4), immunoglobulins (IgG, IgA, and IgM), ceruloplasmin (Crl), and transferrin (Trf) were determined weekly for nine septic postoperative patients, all of whom had multiple organ-system failure. The peripheral blood total lymphocyte count, its subpopulation, T-cell subset, and proliferative responses of lymphocyte to phytohemagglutinin (PHA) and concanavalin A (Con A) decreased in all patients. OKT3 and B-1 decreased progressively in the four nonsurvivors compared with the five survivors. Although immunoglobulin levels were within the normal range in both groups, they tended to increase in survivors and decrease in nonsurvivors. Serial levels of C3, C4, Crl, and Trf increased in survivors but did not change in nonsurvivors. T-cell function and antibody-producing activity diminished progressively in nonsurvivors. These changes in cellular immunity may represent another manifestation of multiple organ-system failure during sepsis.