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Cells Peripheral blood mononuclear cells were separated from heparin-ized normal human blood by Ficoll-Conray density-gradient centrifugation and were washed and resuspended in RPMI 1640 medium supplemented with 10 percent heat-inactivated fetal bovine serum (GIBCO), 2 mM L-glutamine, and gentamicin (60ngfail) (complete medium). The PBMCs were then depleted of adherent cells by plastic adherence as described previously. The remaining cells, referred to hereafter as PBLs, were washed and resuspended in complete medium. The viability of cells was assessed by the trypan blue dye exclusion test. Cytotoxic Assays The NK cell activity was assessed through the standard s‘Cr-release assay. Briefly, 3 X 105 effector cells (PBLs) and 1 x 104 5,Cr-labeled K562 target cells (unless otherwise stated, the effector-target ratio was at 30:1), an erythroleukemia cell line, were incubated in U-bottomed microculture plates in a total of 0.2 ml. Then those plates were incubated at 37°C in a humidified 5 percent C02 incubator for 4 h. The percentage of specific lysis was calculated by the following formula: percent lysis = (experimental cpm — spontaneous cpm)/(maximum cpm — spontaneous cpm) x 100. All experiments were performed in triplicate. The spontaneous release was less than 10 percent in all experiments.